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Priya Koothrappali is Raj's younger sister. She first appeared in "The Irish Pub Formulation" and serves as the unintentional main antagonist of season 4 in that Penny regards Priya as Leonard's most viable alternative girlfriend.
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 Amy, though she has difficulty keeping up any lies she gives. Priya also called her "interesting," and would frequently invite her and Howard to double-dates.
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 frequently insults Priya; however, she admitted Priya had a lot of good qualities that Leonard was attracted to, such as her intelligence and beauty.
- Priya graduated with a degree in law from Cambridge University in the United Kingdom.
- She can practice law in three different countries: most likely the UK (where she studied), India (her birth country), and the USA (where she worked at the time).
- 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 Stephanie Barnett.
- Priya is the only girlfriend Leonard had who Raj can talk to or around without being drunk, obviously for family reasons. He talked to Penny in the Season 6 finale (without being under the influence), but this fact doesn't mean he will be able to constantly talk to her in Season 7.
- 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 4 due to her relationship with Leonard creating a 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 time frame is how long he and Penny originally dated before they broke up.
- Throughout Priya's appearances it can be seen that Penny may be jealous of Priya, even agreeing to spy on her with Amy and Bernedette.