Rajesh Ramayan Koothrappali is Wolowitz’s best friend and yet another genius. He is of Indian origin but despises Indian food. He has a thick Indian accent and, like Wolowitz, a love for Stephen Hawking. For comedic affect, Raj is commonly left on his own in Leonard’s and Sheldon’s apartment without any acknowledgment by his friends. He works in the Physics department at Caltech, where his area of expertise is particle astrophysics. For his discovery of a planetary object beyond the Kuiper Belt (which he named “Planet Bollywood”) he was included in People magazine’s “30 Under 30″ to watch, and thus received a larger office for his work, granting him some form of celebrity status. However, the work Raj performs at the University is repetitive enough that he could conceivably get replaced by a simple software program.

He approaches women in the exact opposite way to Wolowitz: he is so shy that he is unable to speak to them unless he is intoxicated or does not feel he is directly addressing them, however, he is able to speak around female family members, if the women are part of a crowd or if he is unaware of their presence. His social anxiety around women seems to be indicative of selective mutism, and is inspired by a former coworker of Bill Prady’s. During Season 1, Penny cornered him to talk to him about her frustration with Leonard and Sheldon, he voiced a lengthy inner monologue, reminiscing of his native customs and singing. More often than not, though, he just stays silent. Alcohol appears to suppress his social anxiety, since he has prolonged conversations with women after drinking. He first discovers this when Penny mixes a “Grasshopper” cocktail for him. This has caused him to expect that the consumption of alcohol will allow him to speak, thus making him susceptible to the placebo effect. Raj can talk to women even if he thinks he’s drunk (as shown when he drinks non-alcoholic beer while talking to Summer Glau). In the episode the Griffin Equivalency, Raj is able to talk to Penny as he has become arrogant and full of himself, so we can assume that his over confidence canceled out his mutism. However after being a total jerk to Penny while drunk, he ashamedly reverts back to his old persona. He tries to apologize through a note. Penny refuses saying she wants an actual apology with words. He manages to utter a very high-pitched “Sorry”, which is accepted. An experimental anti-anxiety drug enabled him to talk fluently to women, but had side effects on his motor controls and mannerisms. Eventually, the drug wore off at the wrong moment: when Sheldon’s sister Missy, who had rebuffed Leonard and Wolowitz, admitted that she was attracted to Koothrappali and had been hoping he would ask her out. Raj thinks about someday marrying an Indian woman so she can sing lullabies from his native land to his future children.

Quick Facts

• Whenever Raj gets an idea or wants to say something slightly controversial in a conversation, he whispers it in Wolowitz’s ear. Wolowitz never wants to say it out loud because it finds it stupid or offensive, but he ends up saying it anyway through scoffing Raj.
• He regularly communicates with his parents (V. M. Koothrappali and Mrs. Koothrappali) back in India via webcam. When they become meddling or judgmental, he often “brings his parents” into Leonard and Sheldon’s apartment via his MacBook Pro, to seek his friends’ support in the discussion. His father is a gynecologist.
• Rajesh often has married-couple-like arguments with Wolowitz and when nervous often whispers into his ear leaving him to translate to everyone else.
• As with Wolowitz, the others in his circle of friends often refer to him by his surname. He frequently wears a purple jacket and a sweater vest.
• Rajesh preforms vocals when playing Rock Band, which could possibly be a joke because he suffers from selective mutism.
• He despises Guitar Hero because he feels it is lame.
• Raj is considered the attractive one of the group because women find him foreign and exotic, plus his selective mutism makes him a ‘good listener’

Source: Big Bang Theory Wikipedia