Detective "Rosa Diaz" (name is possibly assumed) is considered "the tough guy" and the "badass" in the precinct. Rosa is scary, smart, secretive, and difficult to read. Most of the members of the precinct are frightened and a little disturbed by her. Rosa takes pride in the fact that no one knows much about her. In total, Jake only knows three facts about her, one of which is that she wont let people crash at her place, despite the fact that Rosa refers to him as her "closest friend".
Rosa Diaz is a smart, tough and mysterious detective. Rosa previously attended a parochial school where she was a model student and had a favorite teacher, Sister Bernadette. She also studied ballet at the American Ballet Academy but was kicked out for "beating up the ballerinas". She mentions in The Mattress that her ballet teacher, Ms. Miriam, used to complain to her about her "Posture, technique, and pink hair." The pink hair may be a reference to Stephanie Beatriz's hair, which sometimes has pink highlights in it. Rosa was eventually a classmate of Jake Peralta who was in the police academy, where the two became friends.
She is a recipient of the US Flag Bar, N.Y.P.D. Exceptional Merit, and N.Y.P.D. Firearms Proficiency Bar. In season two, Diaz believed she was pregnant with Marcus' child, until it is revealed she was not. She and Marcus break up in Into the Woods and reveals to Jake that her current boyfriend is called Tom in The Swedes. It is presumed that she and Tom broke up some time between The Swedes and Cheddar, where she gets together with Adrian Pimento. She and Pimento break it off eventually, and she starts dating an unnamed woman.
Her family comes from Bensonhurst and her father is a teacher. She has two sisters, a brother named Sam and a niece named Diana.
She has curly, dark hair and often wears a leather jacket and black jeans or 'badass' clothes. She has dark, revengeful eyes and also has a thin cut in her right eyebrow from when Beatriz tripped on a Lego brick when she was 10. She often has an offensive stance, with her arms crossed and a scowl on her face.
In Pilot, Rosa is introduced as tough, smart, hard to read, and really scary. It is also revealed that Charles Boyle has a crush on her, despite not being her type at all. Charles, Rosa, Jake and Amy assess the scene at the murder victim Morgenthau’s apartment. Rosa and Charles are sent to check in with the coroner. On their way there, Charles asks Rosa if she’d like to join him at an old movie festival. She agrees, but when he suggests watching Citizen Kane, she shows dislike for the film and suggests he finds something better for them to watch. She and Charles go to the coroner but get no clues from him. Later, Rosa approaches Charles to ask him about the film they will be seeing. He claims to have bought tickets for several different films, insinuating that she would be difficult to please if he was left to make the decision himself. This angers Rosa and she cancels the plan altogether. Later, Rosa and Charles sit in a different vehicle. Rosa offers Charles money to pay him back for all the movie tickets he had sadly purchased. She refuses to watch a film with him, but admits she enjoys his company.
In The Tagger, Amy, Rosa and Charles are assigned to a drug bust and Gina introduces her friend Carlene, a psychic who offers to help them with their case. Amy and Rosa are sceptical, insulting Carlene’s so called gift, and leave. Charles is told by Carlene that Rosa will never love him back and become upset by this. Later, Amy, Rosa and Charles weigh their confiscated drugs in the evidence room as Gina watches. When Amy steps out, Rosa and Charles discuss their plans for the evening, discovering they are both free. However Charles doesn't ask her out due to Carlene's warning. Rosa discovers Charles rolling around the office in his chair and he informs her of the psychics new prediction. As a result, she punches him in the arm and claims that he’ll get hurt if he stays in the chair, too. She tells him to ignore the hack psychic and make his own destiny.
In The Slump, Amy and Rosa are trying and failing to engage the youths in the Junior Policeman Program for at-risk youth. Rosa fails and they even make a remix of her voice saying: "Throwin' you in jail, throwin' you in jail, got it, got it?" She also remarks that she does not recognise the feeling of being laughed at. Amy and Rosa are forced to turn to their last hope: Gina. She had previously asked them if she could help, but they had turned her down. Gina manages to convince a few to join and tells Rosa and Amy as she leaves that the programme doesn't work as she was in it.
In M.E. Time, Rosa and Jake are assigned to be Charles' secondaries on a case he's working on. Jake tried to take lead and Rosa tells him that he needs to be take a step back. Later Jake tells her and Amy that he had sex with Dr. Rossi. Rosa is shown to be disgusted by Jake's date with Dr. Rossi and thinks her corpse fetish is gross. Later when Rosa and Charles go to find Jake after he went to get the autopsy and they find him in a body locker and Dr. Rossi half dressed.
In The Vulture, Rosa is shown to hate the Vulture but also be attracted to him much to Charles's annoyance. She goes with Jake and the others to the crime scene and helps to solve their current case. Jake later takes the blame for going onto the crime scene illegally and Rosa doesn't get into trouble.
In Halloween, Rosa helps Jake to steal Holt's medal of valour by picking the lock of his cabinet whilst Jake created the distraction of Herman the Janitor. She is shown with the others laughing when Holt says that Peralta is an amazing detective/genius.
In 48 Hours, Rosa and Gina have a domestic about whose pie is better. Rosa's pie is actually disgusting, but as Charles has feelings for her, he lies and says hers is better when asked to judge. Rosa is annoyed when it is revealed that Charles lied, but her and Gina are impressed by the pie that Charles brings in.
In Old School, Terry and Charles give Rosa a crash course in how to be appropriate in court after several bad past experiences. They teach her phrases and find her some appropriate clothes for court. However, Rosa is nervous and messes up which makes her seem like a 'psycho'. Charles tells Rosa to go to her happy place which she does and they find the suspect guilty.
In Sal's Pizza, Amy is surprised to find out that Rosa was offered a promotion to be captain of Ropesburg PD as Amy wants to be captain. Amy continually tries to get to the bottom of why Rosa was offered the promotion and not her which annoyed Rosa. Rosa then drives her over to Ropesburg where it is revealed to be very boring. Rosa tells Amy they need to work together as they are the only female detectives in the precinct.
In Thanksgiving, Rosa goes to Amy's thanksgiving party as she is tired of 'perfect' thanksgivings with her sister and wants the holiday to be train wreck. As the dinner gets worse and worse, Rosa enjoys herself, being amused by the misfortune. Later, Rosa says she was having a great thanksgiving until Charles saved it.
In Christmas, Amy and Gina attempt to get Rosa to smile as they are making a card for Holt. Rosa doesn't smile when Hitchcock tries to tickle her, instead she attacks him. Later Amy and Gina attempt to make her smile but they mess up and Rosa smiles but they don't have a camera. Later, Rosa comes with Jake and the others to find the man who is attempting to kill Holt. Whilst there Holt's attacker sees her and tries to kill her. Charles jumps in front which saves her life.
In Pontiac Bandit, Rosa introduces Jake to her perp. Doug Judy after he asks to speak to Jake about the Pontiac Bandit. Doug Judy is shown to have a thing for Rosa as he sings her a song. Rosa and Jake agree to work with him to catch the Pontiac Bandit. Later when a meeting goes wrong, Rosa tells Jake that something doesn't seem right and they shouldn't let him go in alone. Jake assures it's okay and she begrudgingly goes along with it. Rosa gets annoyed when Doug Judy escapes as Jake should have trusted her. Later Jake officially adds her to the case and Rosa makes him do 2000 push ups as way of apology.
In The Bet, Rosa feels obligated to be nice to Charles as he saved her life. Gina helps her avoid him when he has too much pain medication as she thinks he's going to tell her he loves her or propose and she doesn't know how to handle it (which makes Gina want to cry.) Later he finds her and tells her that when he saved her life, he didn't know it was her. He only did what any good officer would do and if she goes out with him, he wants it to be because of something only he would do.
In The Ebony Falcon, Rosa and Amy are asked to investigate a theft at Gina's house. Although they initially make fun of her for overreacting, they later realise she is scared and they help her make her apartment safer.
In Operation: Broken Feather, Rosa and the others are controlled by Terry and Holt to make them work more effectively. Rosa is deliberately made angry so that people won't go past her and bathroom breaks will be cut down on. Rosa is later shown to be annoyed about them especially as they called her "Mount Diaz".
In The Party, Rosa is invited with the others to Holt's party. She is given the task of babysitting Gina to make sure she doesn't steal anything. Rosa introduces Gina to some psychologists and they are very intrigued by her.
In Full Boyle, Rosa and Amy don't take the dressed up, Super Dan seriously and won't take his statement. Later they are annoyed that they are removed from the case and ask Terry why. He explains that they were rude to Dan and he won't work with them. He also reveals that Dan has a lot of evidence that would be crucial to the case. They apologise to Dan and he agrees to work with them.
In The Apartment, Rosa is annoyed that her weekend deskmate, Lohank keep shaving on her desk (especially as her "R" key is jammed with stubble). She and Charles seek revenge and fill his locker with shaving cream and hair. They have a nice time together and enjoying hanging out now that Charles is not in love with her. Late after hearing Lohank's sad story they run off to clean up his locker.
In Tactical Village, Rosa is annoyed when she finds out that Charles didn't give her a save the date for his wedding as she thought they were friends. Throughout the training, she attacks Charles with numerous weapons. Later, after she nearly "dies" in the simulation, she decides to tell Charles that is mad. Charles reveals that Vivian thought it would be awkward but he later gives her an invite. She later speaks to Vivian who reveals that it was Charles that thought it would be awkward and didn't invite her.
In Fancy Brudgom, Rosa offends an officer after he misfiles a weapon. She humiliates him but agrees to drop the complaint if she apologizes to him. She does but it's not sincere and he doesn't accept it. Later she goes and sincerely apologizes as well as offering to show him how to file the weapons correctly.
In Unsolvable, Rosa finds Charles in the storage cabinet calling Vivian as there is no privacy there. Rosa shows him her and Gina's secret bathroom called "Babylon". Charles is grateful but is threatened by Hitchcock and Scully and tells them where it is. Although Gina is cross, Rosa stands up for Charles saying that he needed it and also his life is falling apart.
In Charges and Specs, Rosa and the others try to comfort Charles after his breakup with Vivian. She shows him her technique of setting things on fire to help ease grief but it just ends up burning him. Whilst testifying for Jake, she is told to stall and speaks very slowly with large pauses between each word. Later at the bar Charles is still upset and she reveals that the reason she never cared for her breakups is that they were all jerks, not like Charles. When Jake is sent undercover, he leaves Rosa his locker before leaving.
In Undercover, Rosa is happy to see Jake back and welcomes him back to the precinct. Rosa is shown to be annoyed by the continuous drills that Captain Holt is making them do. Rosa and Amy take Terry who is pretending to be a little boy onto a bouncy castle. Holt gets annoyed with them and tells them to stop.
In Chocolate Milk, Rosa is annoyed when Charles asks her to be his plus one to his ex-wife’s engagement party. Eventually she agrees to go and also says that she will help him move out of his ex-wife’s basement.
In The Jimmy Jab Games, Rosa competes with the others in the Jimmy Jab games. Jake challenges her that if he wins then she will give him her friend, Katie’s number and if not, he will give her $200.. After Rosa goes out she calls Jake over and asks if he wants to call off the bet. He asks her if it’s because she’s losing and she says it’s because he’s still into Amy. He dismisses this claim and she tells him to prove it by not flirting with Amy at all. When Holt comes in and asks what is going on, she tells him the truth. He manages to but loses. Rosa tells him that she is worried about him as it’s not like him to get hung up on a girl. She gives him Katie’s number to move on. Later she is asked to run the drug task force.
In Halloween II, Jake enlists Rosa’s help to help him win the bet. She is given the nickname “Dagger” and she takes out Holt’s doorknob. Later, it is revealed that she was on Holt’s team and distracted him on the party bus whilst Holt stole his badge.
In The Mole, Rosa and Terry visit a silent disco as they believe there will be drugs as they haven't found any major drug dealers yet. Whilst there they meet a girl called Ava Watson who claims to not drink nor drugs. Rosa is annoyed that Terry is preoccupied with his daughter's school rather than the case. They then see Ava dealing drugs. The interrogation brings about very little and Rosa is annoyed. Later she is happy when Terry finds out that Ava is actually called Rebecca Lubbock and they now have leads on a possible supplier.
In Jake and Sophia, Rosa attempts to pressure Amy into running for Union Rep as she is convinces that Amy will make a great one. Amy is annoyed at her but eventually decides to run and Rosa is proud of Amy's speech.
In Lockdown, whilst in lockdown, Rosa relentlessly teases Gina about her fear of dying due to the suspicious package. Jake later tells her that he needs Rosa to help Gina instead of teasing her and making her feel worse.
In USPIS, Rosa tells Jake that he has to work with Jack Danger from USPIS to help solve the drug case. When Jake manages to solve the case without USPIS, Rosa is angry as Jack complained and now anything that happens in the case will go to USPIS and sarcastically thanks him.
In The Road Trip, Rosa is sick from her cold but refuses to admit it. Terry and Gina try and help her but she won't let them and continues to want to carry on with her case. Rosa takes too much cough medicine and becomes dazed. Terry then gets Gina to distract Rosa whilst he interrogates her perp. Rosa is annoyed that Gina locked her inbut then sees that they're trying to help and accepts it.
In The Pontiac Bandit Returns, Doug Judy is arrested by her and Jake. After he says that he can take them to a major drug leader, Tito Ruiz, they cut a deal with him. Whilst in Doug Judy's hotel room, Judy flirts with Rosa and she is forced to be nice due to him helping them. Rosa later comes with Jake and Doug Judy to meet Tito, posing as Doug Judy's bodyguard and lover, Celeste. After Jake and Rosa go to Tito's drug ring, Doug Judy's associate bursts in. Jake nearly arrests him but at the last moment chooses to arrest Tito instead. Rosa thanks Jake for catching Tito not Doug Judy and assures him that they will catch him. She then smiles happily due to the success of the day.
In Stakeout, Rosa meets Holt's nephew, Marcus and is immediately attracted to him. Holt tells her that Marcus asked if she was single and Rosa says she is but if Marcus wants her number he has to ask her for it personally. Rosa and Marcus later come out of Marcus' room in front of Holt and Kevin. Rosa finds it awkward and tells Marcus that she shouldn't see each other again. Later Holt speaks to her as a friend and tells her that if she wants to see Marcus, she should. Rosa says that she might do but doesn't want to talk to Holt about it. They both agree to never talk about it.
In Beach House, Rosa lets Marcus kiss her goodbye and she is annoyed when Charles comments on it. Rosa is annoyed when Jake invites Holt. She says that the weekend will suck now. She later asks Charles for help in texting her boyfriend as she has accidentally hurt his feelings. Charles helps her which culminates in her and Marcus exchanging shirtless photos.
In Payback, Rosa is confused when Jake suddenly pays Terry back and demands to also be paid back. Instead, Jake offers to do her favours and she makes him wash her bike. Later, she finds out about Terry having a baby when Jake accidentally replies to all of them on an email. She helps delete the email off of Terry's phone with the others.
In Defense Rests, Holt asks Rosa's advice on how to deal with Madeline Wuntch as Wuntch needs his help for a promotion and he doesn't know whether to help her or sabotage her. Rosa tells him that revenge doesn't pay off and he should take the high road.
In Windbreaker City, Rosa and Amy both want to have the day off at the same time as Amy wants to go to a ted talk and Rosa wants to have dinner. They decide to settle it by seeing who can shoot the most people. Although Amy wins, Rosa gets the day off as she tells her that she is meeting Marcus's parents and that Amy doesn't need to learn about power poses.
In The Wednesday Incident, Charles brings in an old man, Marvin Miller as his suspect for a heist. Rosa and Charles doubt him and end up being nice to the geriatric, including having a tea party with him. After he dies, Rosa and Amy find evidence that he was guilty and congratulates Charles.
In Boyle-Linetti Wedding, Rosa helps Lynn Boyle with his cufflinks and accidentally makes him nervous. She later realises she was being a coward for not inviting Marcus and taking things faster and invites him. The two say that they love each other and kiss.
In Captain Peralta, Holt gives the squad an impossible brain teaser. Gina and Rosa pair up but can't work it out. Later, Rosa is impressed when Gina figures out that Holt doesn't know either.
In Sabotage, Rosa and Amy got to investigate Jake's sabotage claim. They tell him to go as he is suspended and he angrily does. Later, they figure out that he was right and rescue him.
In AC/DC, Rosa and Marcus go for dinner with Holt and Kevin. Rosa wants to avoid awkward conversations with him and avoid getting to know him too well. Later, Holt is forced to help Rosa when she worries that she may be pregnant. Later, she thanks him for helping.
In Det. Dave Majors, Rosa advises Jake on what to do about Amy. She tells him that she doesn't ask people out, she tells them where they're going. Rosa tells Jake that to give Amy a choice, then he should ask her out and give her him as an option.
In The Chopper, Rosa helps Terry with a school trip from the local magnet school. After Amy fails to keep them entertained, Rosa shows them a dead body which horrifies the teacher, Nancy Sussman. She leaves but Rosa and the others convince her that it wasn't a complete disaster.
In Johnny and Dora, Charles accidentally reveals that he is in charge of bringing Rosa to a surprise party. Rosa is cross and eventually bans him from even calling her Rosa as she believes that he doesn't know her. However, after the party turns out to be just her and Marcus in an empty bar, which she loves, she forgives Charles and even reluctantly lets him call her "Ro-Ro",
In New Captain, Rosa waits with the others for the new captain to arrive. She is shown to dislike Seth Dozerman, the Captain and is annoyed at the Dozer-pads. He tells them that he couldn't remove Backgammon from the pad and tells them they are not to play it. Rosa says that she is going to play it just to annoy Dozerman. Terry becomes annoyed when she won't follow Dozer's orders, even putting the Dozer-Pad in the ceiling. After Dozerman's death, she is shown to feel some sadness and eventually follow's the Dozer-Pad's schedule.
In The Funeral, Rosa attends Dozerman's funeral with the rest of the precinct. She and Gina are disgusted that Charles' is going to hook up with a police officer without even knowing her as it is out of character for him.
In Boyle’s Hunch, Rosa and Terry team up to catch Hitchcock and Scully stealing their snacks after their Ice Cream and yoghurt go missing. They even go as far as to get the spoon tested to prove that Hitchcock and Scully are guilty. After Scully says that is can't be them, as he is lactose intolerant, Rosa and Terry buy them a pizza to trick them. After Hitchcock and Scully eat the pizza, Rosa triumphantly exclaims that it was a trap as Scully is eating the pizza. Scully confesses that him and Hitchcock did steal the ice cream and yoghurt and then says that they are now eating their pizza, much to Rosa and Terry's annoyance. Hitchcock and Scully then get stomach troubles and have to go to the bathroom. Terry says that at least they won but Rosa says there are no winners.
In The Oolong Slayer, Rosa and Amy are asked by the Vulture to plan his birthday party. Rosa mocks Amy for actually planning their Captain's birthday seriously especially when Amy goes onto his facebook to see his interests. However they discover that he is in a band and Amy suggests they hire him to play at his own party as everything he does is mocking himself.
In Halloween, Part III, Jake enlists the help of Rosa and Charles which upsets Amy who is not included. Rosa successfully steals the crown from right under Gina's nose by breaking into the interrogation room and making it out again in under a minute; however Terry distracts Rosa whilst Holt steals it back. After Jake discovers that a Janitor named Al had the crown, Rosa goes with Charles to steal the crown back. After finding out that it is Amy who has the crown, Rosa is impressed as Amy reveals how she did it.
In Into the Woods, Captain Holt that Rosa wants to break up with his nephew, Marcus. He gets Rosa to practice her breakup and although it is a cold breakup, both of them think it is fine. Holt later calls her back to his office and tells her that Kevin said that her breakup wasn't okay and said they were both 'Sociopaths'. Holt looks online and suggests that Rosa acknowledge Marcus's feelings. Rosa later breaks up with Marcus and after he gets upset, she attempts to climb out of the window. However Marcus calls her back and she has to endure his emotions. She tells Holt that she did care about Marcus but he wanted to get married and she didn't and she was worried that she had sabotaged her one chance at love. She and Holt then both become emotional about the situation and cry.
In The Mattress, Terry finds Rosa talking to a boy called Sam and tells her off for calling him 'Stupid'. Rosa explains that Sam is her assigned 'Little Brother' and Sam thinks it's funny when Rosa calls him stupid. Rosa tells Terry that she loves being a mentor. Later, Sam comes into the precinct after being caught for theft. Rosa lashes out and tells him she will make sure he rots in jail. Terry calls her away and attempts to get her to be kind to Sam and let him off with a warning. Terry then brings in her old ballet coach, Ms. Miriam to show her the effect a kind, nurturing presence can have on a child. However, Ms. Miriam proceeds to be mean to Rosa and Rosa explains that Ms. Miriam was not nurturing. Ms. Miriam then forces Rosa to ballet dance, saying that it wasn't terrible. Later, Rosa reveals that she left Sam off with a warning as Ms. Miriam made her good dancer but she also made Rosa quit ballet. She doesn't want to be the reason that Sam leaves the programme so decides to be nice.
In Ava, After Terry finds out that Sharon is going into labour, Rosa agrees to help Terry get back as fast as possible. She takes him on her motorbike and gets him to the hospital. Later, Terry thanks Rosa for getting him there so quickly.
In The Swedes, Jake and Rosa are forced to work with two detectives from Interpol, Soren and Agneta. They instantly take a disliking to the Swedes despite this annoyance not being reciprocated. Rosa and Jake go to the crime scene and are annoyed to find that the Swedes have already started working without them. Jake finds a receipt they missed and Rosa and Jake go to look for the diamonds but don't find anything. Jake asks Rosa to help him with an idea for his and Amy's anniversary but she doesn't want to. Jake is also upset to find that Rosa broke up with Marcus as he thought they were friends and friends should tell each other things. Later they realise that they read the date wrong as Europeans write it differently. They go to the right container and find the diamonds but the container is suddenly driven away. Jake gets annoyed at Rosa and they break friends. Jake and Rosa manage to catch the diamond criminals and go for a drink in silence to celebrate. Rosa admits that she would love to drink in silence but she can see that Jake is sad and that sucks because they are friends. She tells him what to do about for his anniversary and reveals she has a new boyfriend, Tom.
In Yippie Kayak, Rosa and Holt decide to go for a polar swim. Amy is upset that she wasn't invited and asks to come along, despite Rosa and Holt warning her not to. At the polar swim, Rosa and Holt go into the water but Amy doesn't as it is too cold. Later, Amy runs in to get them out as Jake is in a hostage situation.
In Hostage Situation, Rosa and Holt are baffled when one of the people they have arrested agrees to talk but only to Gina. It is revealed that he knew Gina in high school but she doesn't recognise him. Rosa and Holt reluctantly let Gina go back in to interrogate him but she just wants to find out about people from their high school. Later Rosa gets him to talk by complaining about Gina.
In 9 Days, Charles's dog, Jason has died and Rosa mocks him for caring so much at first. She later attempts to help him by getting him a new dog to replace the one that dies so he can get back on with his work. Charles is upset at this and leaves. She later calls him back and agrees to host a proper memorial for Jason. This is because she now understands why people care for pets as if anything happened to her dog Arlo (the dog initially bought for Charles), she would kill everybody and then herself.
In The Cruise, Rosa and Charles compete for the apartment of the victim of their murder case. They both start to butter up the landlord Mr Gotaro in attempt to get the apartment. Mr Gotaro thanks them but tells them that he has decided to rent to somebody else. Rosa and Charles later find him guilty of murdering his tennant and after finding out that the apartment is up for rent again, decide to compete for it again. Rosa is also mentioned by Doug Judy in a song called 'Rosa, Rosa, Rosa'.
In Karen Peralta, Rosa and Charles go into a crime scene with new body cameras. However whilst she is in there, she enters the crime scene to find Charles completely naked much to her horror. The entire incident is caught on her camera. They find out that there is not enough evidence to convict their suspect of having cocaine, but Terry is happy as they have evidence on their body cameras. Rosa and Charles protest but Terry forces them to show the footage. They get the evidence to convict their suspect but the police officers are also forced to watch the naked Charles incidence.
In The 9-8, Rosa is annoyed when the entire 9-8 precinct is forced to come and stay with the 9-9 after problems at their precinct. She is annoyed by her desk mate Ellen, who has lots of dolls and enjoys looking at recipes only to say 'Who's got the time.' Rosa later starts to chant 'Juice Ellen!' in anger and Terry takes her and Amy outside. They decide to move their equipment outside to escape the 9-8 but Holt puts an end to it much to their annoyance. After a fight breaks out, Rosa joins by smashing one of Ellen's china dolls on the floor.
In House Mouses, It is revealed that Rosa is afraid of needles and won't donate blood due to this. Amy and Gina decide to help her get over her fear by facing their own fear. Amy stays in a car boot to get over her fear of small spaces and Gina gets over her fear of business people by sitting and mingling with them. Rosa decides to face her own fear and donates blood after seeing the effort the others put in.
In Adrian Pimento, A new detective, Adrian Pimento arrives at the precinct after being undercover for 14 years. Rosa is attracted to him and warns Jake that this means something is wrong, as she is only attracted to creeps. She later helps Jake by keeping watch on Adrian but later finds out that Adrian was not working for criminals.
In Cheddar, Rosa and Adrian have began strangely flirting which makes everybody uncomfortable, including the criminals. Rosa, Adrian and Terry are sent to go put up missing posters for Cheddar. Rosa suggests that she and Adrian begin a relationship but he freaks out and leaves.Rosa gets angry and angrily staples a missing poster for Cheddar. Later, after talking to Terry, Adrian proposes to Rosa. She rejects him but agrees to go on a date with him instead.
In Terry Kitties, Rosa and Holt attend a bomb diffusing class. Amy invites herself along and Rosa and Amy become competitive. Eventually, Holt tells them to calm down but it is revealed that he played them both so he could win. They all are annoyed, however, when they find out that Hitchcock and Scully beat them all.
In Paranoia, she became engaged to boyfriend Adrian Pimento, who proposed to her while chasing a drug dealer through an alley. Terry is worried about her relationship with Adrian and Rosa asks Jake to help her get Terry to approve of her relationship with Adrian. Jake agrees and accepts Adrian's offer to be best man. Rosa asks Gina, Amy and Charles to be her join maids of honour. All three of them give her a bachelorette party so Rosa can choose a winner. Gina takes her paintballing, Amy creates a quiz/drinking game and Charles lets her destroy a building. She says that Charles's bachelorette party is the winner. However before she can get married to Adrian, Adrian discovers that a hit man is coming to kill him and is forced to leave after faking his own death.
In Maximum Security, Rosa is chosen to go undercover in a woman's prison as Jessica Cortez. However upon entering she is recognised as a cop and her cover is instantly blown and she has to leave. She helps Terry and Holt plan a fake funeral for Adrian and is annoyed that she has to play the grieving widow. She makes a sad speech about him and states that he's gone. Terry however assures that she is wrong and they will get him back.
In Bureau, Rosa and the others team up with Bob Anderson from the FBI to help discover the mole in the FBI. They manage to get the file which says who the mole is and they track down the mole. However, when they find him they see that he has been attacked and is left for dead. They manage to get him to hospital and Jake and Rosa leave to go and get a drink. Amy then calls them to tell them that there are two moles in the FBI, Bob is the other one.
In Greg and Larry, Rosa and Jake rush back to the hospital and see that Holt and Bob are missing. They figure out that he is on the helipad and go up and together, they manage to save Holt. They all leave the hospital disguised as workers and Rosa says that she will let them use her house as safe house. Everybody is amazed at how nice Rosa's house is as they were expecting her to have a much darker apartment (Jake was expecting a lair). Rosa helps trick Bob into thinking his life is at threat and she and Jake go and discover the real file containing the moles.
Rosa Diaz is a smart, tough and mysterious detective (it is revealed that her name might not even be Rosa Diaz, as she assumes multiple identities for different people). Although Diaz is a loyal and effective member of the squad, her anger management issues and lack of empathy terrify most of her coworkers. She is highly secretive about her personal life, and refuses to let any of her coworkers visit her home, despite living very close to the precinct. She is shown to own many weapons and believes that every woman should own an axe. Rosa also owns multiple swords.
Rosa and Gina have a strange, on and off friendship. Whilst they do get along in some episodes, Rosa has also been shown to mock Gina in several episodes, such as when Gina is afraid that she will die of Anthrax.
Jake and Rosa are good friends from the Police Academy and are shown to trust each other immensely. In Pontiac Bandit, it is revealed that Jake and Rosa have a pact called '1000 pushups' which means they have to trust each other. Jake breaks this trust but later makes it up to Rosa by actually doing 1000 pushups. In The Swedes, Rosa said that Jake was her closest friend in the world as they don't talk about personal issues. By the end of the episode, the two have developed a closer relationship as Rosa is more comfortable talking about personal issues.
Rosa and Amy have a friendly relationship and seem to be on good terms. In House Mouses Amy helped Rosa get over her fear of donating blood which shows that the two have a friendly and supportive relationship. However the two have shown rivalry such as in the bomb diffusing class and when Rosa gets offered to be the captain in Ropesburg.
Rosa and Charles previously had an uncomfortable relationship as Charles was in love with Rosa but it is not reciprocated. After Charles and Vivian start dating, Charles and Rosa become friends and Charles no longer has feelings for Rosa. They are shown to be friendly such as in Johnny and Dora, Charles plans a birthday party for Rosa that she likes and she lets him call her 'Ro-Ro'.
Rosa and Holt have a close, impersonal friendship. Both are uncomfortable with personal situations and avoid discussing personal matters. Things become complicated when Rosa begins dating Marcus, Holt's nephew but they manage to avoid discussing personal matters and even manage to help each other which brings them closer.
Rosa and Terry have a friendly relationship. They are shown to be working together such as when catching Hitchcock and Scully who are stealing their snacks. Terry is also shown to be helpful to Rosa by teaching her how to be good in court and kinder.
Rosa and Adrian have a romantic relationship that makes most people, especially Terry at first, feel uncomfortable. In Cheddar, the two are shown to be aggressively flirting and making the entire precinct uncomfortable. The two get engaged but their relationship is halted when Adrian has to fake his own death and leaves. Rosa is shown to be very upset by this but confident in his return. She is also very pleased when he comes back shown by violently making out. Rosa accuses Adrian of cheating on her, but she is wrong. She dumps him a while later.
Rosa and Marcus previously had a romantic relationship despite having very different personalities to one another. Rosa felt uncomfortable due to the fact that Marcus is Holt's nephew and doesn't want to complicate her relationship with her boss
Some of the movies that Rosa enjoys are Something's Gotta Give (which she found to be hilarious) and RoboCop. Gina also told Boyle in the "Pilot" that Rosa enjoys old movies, but he later found out that she hates Citizen Kane.
Rosa uses bolt cutters to make jewelry.
Rosa is shown to be dark and aloof, but cares for her friends, as shown when she asked Gina, Amy and Charles to be her co-maid's of honor.
As of Paranoia, she's never had soup and her first word was "Da-da",
She does yoga because it helps keep her centered. She can also do a handstand.
Rosa said she once had Amy over to her apartment, and moved the next day. 
It is revealed that she lives in apartment 410. Her neighbors think her name is Emily Goldfinch, who has curly black hair, is always smiling and a little chatty, which is the total opposite of her personality. It is also revealed that she rented the apartment from a shell corporation, her mail goes to a PO. Box in Queens and her real name may not be Rosa Diaz. Rosa has most likely moved out, as she was destroying the apartment and she said she was done with the place the minute everyone stepped foot in it.
She loves the director, Nancy Meyers
During Hostage Situation, she seemed very interested in Gina's interrogation of the perp, even urging Captain Holt to let her keep going when he wanted to pull her.
She just wants to see Lorelai Gilmore happy.
She used to do gymnastics and is very agile for that reason. 
Boyle: So where's your happy place? Diaz: I'm in a cabin, in the middle of nowhere. Inside it's just me and that stupid, slimy defense attorney, and I'm beating the hell out of him. I break a dining room table over his head. Then I rip off his arm, and I shove it where the sun don't shine. Then I reach down his throat and shake his hand.
Jake Peralta: I've been thinking about asking her out for the last couple of weeks, but it's hard. There's so much build up now, you know? Rosa Diaz: Not really. I don't ask people out, I just tell them where we're going.