Sheldon is a Caltech theoretical physicist who shares an apartment with his colleague and best friend, Leonard Hofstadter (Johnny Galecki). Sheldon exhibits a strict adherence to routine, a total lack of social skills, a tenuous understanding of irony, sarcasm, and humor, a general lack of humility or empathy, and displays textbook narcissistic behavior. These characteristics provide the majority of the humor involving him, which has caused him to be described as the show's breakout character. Despite speculation that Sheldon's personality traits may be consistent with Asperger syndrome, obsessive–compulsive personality disorder and asexuality, co-creator Bill Prady has repeatedly stated that Sheldon's character was neither conceived nor developed with regard to any of these conditions.
At the outset of the show, Sheldon's closest friends are Leonard Hofstadter, Howard Wolowitz and Rajesh Koothrappali. Of the three, Sheldon is openly dismissive of Howard and constantly opines that a Master's degree in engineering demonstrates a lesser intellect than that of the others, who all possess science doctorates. Despite that, Sheldon has referred to Howard as a "treasured acquaintance" and a friend at varying points. Raj seems to be the most tolerant of Sheldon, and Sheldon also respects his scientific position. Despite this, Sheldon had cut him off as a friend in one episode, but quickly restored the position.
Sheldon is best friends with Leonard, as they live together and have tolerated each other ever since. Prady stated that "the fact that, despite everything, Leonard considers Sheldon his best friend reminds us of Sheldon's essential humanity." Sheldon can handle having only a limited number of friends in his life at a time, but later shows flexibility when he accepts Bernadette and Amy as part of the social group. In Season 6, the guys ask if comic book store owner Stuart can be part of the group while Howard is in space. Sheldon agrees only on the condition that Stuart act like Howard while in the group, and he proceeds to call Stuart "fake Wolowitz."