Priya Koothrappali

Priya Koothrappali is Raj's younger sister. She first appeared in "The Irish Pub Formulation" and serves as the unintentional main antagonist of the fourth season in that Penny regards Priya as Leonard's most viable alternative girlfriend.

Before the series
Priya first met the guys on April 12, 2005. At Bob's Big Boy in Toluca Lake, Howard and Leonard pinky swore not to hit on her. However, Leonard does have sexual relations with Priya. He offers to return home with her to India, but she refuses and leaves.

Priya completed her law studies (LLB degree) at Cambridge University (UK) graduating at the top of her class. She is licensed to practice law in 3 different countries (presumably in the United States, Great Britain and India). She is one of the lead attorneys for the biggest car company in India, having planned an internship at a large Indian car company in 2005 when she was finishing her law degree. She goes to Toronto for a corporate merger and later helps set up a secondary derivative market which would allow overseas car firms to hedge their investments against potential advancements in battery technology.

Season 4
In "The Irish Pub Formulation", Priya is passing through the area and resumes relations with Leonard, sleeping with him in his apartment that night. The following morning, Leonard again expressed desire to join her in India. She refused, stated they've talked about this before and the relationship wasn't serious. They try to sneak out but Sheldon sees them and concludes they had coitus. Leonard returns Priya to the airport and instructs Sheldon to keep it secret. Sheldon further complicates the matter by concocting a crazy story involving an Irish bar maid. When confronted about his night Leonard begins using Sheldon's story, but can't bring himself to say it. He admits that he was with Priya. Raj is angry at first but feels better after hearing of Priya's refusal from Leonard. Following this the guys begin confronting one another about different things they had done wrong to each other such as telling Raj that foreigners give gifts to Americans on Thanksgiving.

In "The Cohabitation Formulation" she arrives on a much longer assignment and starts dating Leonard openly despite protest from Raj. In "The Prestidigitation Approximation", Priya tests Leonard with steadily increasing demands (contact lenses, clothing shopping). In her final request, diminish or eliminate ties with Penny, Priya reasons he shouldn't date while being friends with women he's slept with. Leonard has difficulty telling Penny about it, but she figures it out anyway and agrees to keep her distance. Bernadette, Amy, and Penny band together to vent about Priya's controlling behavior.

"The Agreement Dissection" sees Sheldon accusing Leonard of breaking various roommate agreement clauses. Priya successfully defends on all counts. She continues to poke holes in the contract until Sheldon writes a new contract. He blackmails Priya by threatening to expose their relationship to her parents. Although Priya never reveals why she doesn't want her parents to know, Leonard accepts the agreement at Priya's pleading. It is assumed that her parents do not approve of Leonard or pretty much anyone.

During "The Wildebeest Implementation", Bernadette, on an plot to learn more about Priya, joins Howard, Priya, and Leonard for a get-together. Leonard and Priya mention going to India in the summer to meet her parents. Bernadette asks Priya about parental approval, she states her parents are very old fashioned. Bernadette asks if they're planning on getting engaged, and Priya denies it stating their relationship isn't at that point.

In "The Engagement Reaction', Howard asks Priya why she hasn't told her parents about her and Leonard. She states her parents are extremely protective. Later, in the hospital, Penny and Priya bond while discussing Leonard's sexual peculiarities (long foreplay, dancing). They openly share his quirks with each other much to the discomfort of Leonard.

Leonard moves in with Priya momentarily in  "The Roommate Transmogrification", and Raj moves in with Sheldon so Priya can spend more time with him. She gets a call from her parents while making out with Leonard. She tells him to go to the bedroom while she talks with them. He expresses frustration that she won't tell her parents, but Priya retorts that she'll tell them as soon as one of her brothers or sisters makes a big mistake. Leonard retreats to the bedroom but listens to the conversation. After Priya's mother mentions her return to India the following month, Leonard asks Priya when she had planned on telling him. After her parents ask why she's lying about people in her apartment and Raj being out with friends, Leonard tells her parents about their relationship believing the relationship is doomed anyway. Speechless, Priya walks away from the conversation, leaving Leonard with her disappointed parents. He makes small talk and quickly ends the call.

Season 5
Priya returns to India and, despite parental dissent and physical distance, they continue their relationship with dinner dates and foreplay on Skype (though this is a disaster, as Leonard proves to be very awkward at this type of thing).

However, it's not easy staying loyal. Leonard has a moral crisis in "The Good Guy Fluctuation" where he gets Alice's number at the comic book store. Later at Leonard's apartment, they share comic-con memorabilia and make out. Feeling torn between Alice and Priya, Leonard asks Penny for advice. She states he can't make everyone happy regardless of his choice. Still feeling unsure, Leonard asks Sheldon's guidance. He posits Nietzsche's idea that morality is a fiction used by the inferior to hold back the superior. Taking it to heart, Leonard chooses Alice and leaves for her apartment. But after they make out again, he changes his mind and affirms his loyalty to Priya. Alice calls him a jerk and kicks him out. Leonard, disappointed about Alice but reassured about Priya, returns home, calls Priya, apologizes for making out with Alice, and promises it won't happen again. Priya quickly forgives him by stating all humans make mistakes. Leonard probes her reasoning. Cringing, she admits to sleeping with her ex (probably Sanjay, mentioned to be skyping with her in "The Zarnecki Incursion") a few weeks ago and concludes it's not a contest. Leonard disagrees saying that it is and she won and ends the call. In the next episode, Leonard refers to himself as single, implying that he broke up with Priya afterwards. Later, Leonard starts dating Penny again confirming this.

Priya is never seen or mentioned in the series again. (In the episode "The Recombination Hypothesis," when Leonard daydreams about asking Penny out, Raj mentions his dating of Priya, though he does not mention her by name.)

Personality
Priya is an intelligent lawyer, being certified to practice law in different states. She shows interest in literature and activities such as roller skating. At first she had little interest in dating, as she twice rebuffed Leonard's attempts to date her, but later changed her mind. Priya displays cunning when she unravels the original Roommate Agreement, though she admits defeat when Sheldon blackmails her and Leonard. She does not have much patience with nerdish things, such as when Leonard spent more time playing a game than he did her. Later, he had to claim he was working late so he could help Sheldon with a problem that had come up, knowing she would not be happy about the truth. She also made a lot of demands from Leonard, ranging from the clothes he wore to his relationship with his ex.

Leonard
Prior to the series' start, Leonard and Priya had a brief affair when she visited Pasadena. It did not go beyond it because Priya was uninterested in a relationship. When she visits again, the same thing ensues and her mind has not changed, though she enjoys Leonard's company. It is not until she visits in "The Cohabitation Formulation" that she fnally changes her mind and starts dating Leonard. The two get along perfectly, though Priya gets a little annoyed by Leonard's nerdish activities. In "The Zarnecki Incursion" Leonard is too preoccupied helping Sheldon with a computer game to spend time with Priya, and she goes on to apparently contact her ex-boyfriend.

As their relationship continues, Priya asks Leonard to severe ties with his ex Penny, as she, Priya, feels it is unnecessary for them to be friends. Leonard reluctantly agreed, though the friendship is restored when Priya becomes friendly with Penny.

Priya refused to tell her parents about her relationship with Leonard, preferring to wait for the right moment, and this left Leonard feeling Priya was ashamed of him. At one point, she even threatened to break up with him if he allowed a video about their relationship to be sent to her parents. When she planned on going back to India, she did not tell him about it; he had to find out from her mother, propelling him to reveal their relationship, leaving Priya flustered and her parents angered. Nevertheless, the two continued dating long-distance, though it was difficult. Eventually, in "The Good Guy Fluctuation" Leonard cheated by kissing another girl, and when he confessed, she revealed she had slept with her ex-boyfriend. Leonard, having walked away from temptation, is clearly upset to find that Priya had succumbed much earlier. In the following episode, Leonard confirmed he and Priya broke up. It is unknown whether it was on mutual terms or if Leonard broke up with her.

Raj
Raj is protective of his younger sister, and is very upset when she and Leonard start dating. Priya stays with him while in Pasadena, and Raj is uncomfortable with her sleeping with his friend. He seems to feel better about it over time. It is suggested he may have knew Priya was going back to India, as he did not tell Leonard, though it is possible he himself did not know. It is unknown how he reacted when Leonard and Priya broke up.

Penny
Priya was jealous of Penny because of Leonard's previous relationship with her, and eventually asked him to stop communicating with Penny. Though Penny did so, she became angry towards Priya for dating Leonard, as she had only recently realized she still had feelings for him (mainly becuase of Amy). The friendship is restored when Priya becomes friendly with her, although Penny remains silently jealous. After Priya's relationship with Leonard ends, Penny and Leonard start dating again.

Sheldon
Originally, the two got along fine, with Priya occasionally becoming a bit exasperated with Sheldon's idiosyncrasies. It would be the episode "The Roommate Dissection" that their relationship would sour. Priya, a lawyer, begins to find multiple loopholes in the Roommate Agreement and use them to her and Leonard's advantage. After a while it seems she is doing it simply for the enjoyment of seeing Sheldon flustered. At the end of the episode Sheldon reigns victorious when he blackmails Leonard into signing a new Roommate Agreement by threatening to reveal to Priya's parents her relationship with Leonard, knowing how Priya did not want them to. After this episode, it is shown they are no longer as friendly. In "The Roommate Transmogrification" Leonard reveals Priya no longer spends the night at his apartment because the two don't get along anymore. In "The Infestation Hypothesis," when Sheldon questions Leonard's long-distance relationship with Priya, Leonard points out the similarities with Sheldon and Amy's. Sheldon questions if Leonard likes Amy, and Leonard responds that he does, to which Sheldon retorts "There's the difference," implying their relationship is no better.

Bernadette
While Bernadette did not have any personal dislike towards Priya, she knew how Penny felt about Leonard dating her. In "The Wildebeest Implementation," Bernadette spies on Priya and Leonard for Penny, though she has difficulty keeping up any lies she gives.

Amy
Amy was the first to guess Penny was unhappy about Leonard and Priya (before Penny even did herself), later consoling Penny when the latter started crying. While she frequentley insults Priya, however, she admitted Priya had a lot of good qualities that Leonard was attracted to, such as her intelligence and beauty. She is probably the only person who truly hates Priya, as she frequentley called her a bitch, and had serious thoughts about killing her, and more.

Trivia

 * Priya graduated with a degree in law from Cambridge University in England.
 * She can practice law in three different countries.
 * At Cambridge University, her Texan flat-mate taught her how to make chili.
 * Priya is Leonard's second girlfriend who moved in with him. The first was Dr. Stephanie Barnett.
 * Priya is the only girlfriend Leonard had who Raj can talk to or around without being drunk, obviously for family reasons.
 * Her ex-boyfriend is named Sanjay, as is one of her cousins. Sanjay is also a name adopted by Howard in "The Hofstadter Isotope".
 * Priya can be considered the antagonist of season four due to her relationship with Leonard creating rivalry with Penny. Also, certain aspects of her personality were arguably antagonistic.
 * Out of all of Penny's rivals, Priya was the biggest threat, serving as Leonard's girlfriend the longest, only second to Penny.
 * In "The Skank Reflex Analysis," Leonard mentioned he and Priya had been dating for eight months prior to her returning to India. Interestingly, this is how long he and Penny originally dated before they broke up.