Sheldon Cooper (Jim Parsons) is a massively intelligent young scientist who has absolutely no social skills and manages to offend all those around him with his arrogance and over-pragmatic view of the world. He lives with Leonard Hofstadter (Johnny Galecki), another physicist, but one who is more socially aware than Sheldon. Leonard is desperate for a girlfriend, and often despairs of Sheldon's outlook on life. The longer that they live together, the more he seems to be helping Sheldon develop his social skills. They are both good friends with Howard Wolowitz (Simon Helberg) and Rajesh Koothrappali (Kunal Nayyar), who are both obsessed with women, but, being scientists, struggle to make any headway.
Sheldon has a very straight-laced approach to his life, and his strict adherence to his little routines and rules almost borders on the obsessive. It is this which endears him to his friends, who seem to be impervious to his put-downs and comments. For all his brains, he is quite inept in social situations, and often fails to understand simple innuendo and implied jokes. In the first season, he doesn't understand sarcasm at all, but then goes on a big learning curve, trying it out at any time. Any sarcastic comment that he makes is usually accompanied by him pointing out that he made a sarcastic comment or by the word "Bazinga".
Sheldon is made out to be almost asexual, a fact that is commented on by Penny (Kaley Cuoco) early in the series. Despite this, he manages to attract the attention of at least 3 women throughout the first 3 seasons, but in each occasion, he fails to read the social situation and misses out on the opportunity. He doesn't hold relationships in high regard, however, and sees them as a complicated way of reproducing. At the end of the third season, he is sent on a blind date by Howard and Raj, and meets Amy Farah Fowler (Mayim Bialik), who is essentially a female version of Sheldon. Their relationship is bizarre, with them both obviously enjoying each other's company without ever admitting it, but their plan to have a child (without having intercourse) is stopped by Penny, who calls on Sheldon's mother to talk him out of it.
Whilst "The Big Bang Theory" focuses on the 5 main characters, most of the scenes center around Sheldon and Leonard and their interactions with each other and the world around them. His dry sense of humor and devastating put downs are the central comedy ingredient to the show, and it's hard not to become enamored with him and want him to succeed and develop. His range of phobias and foibles, such as hypochondria and an obsession with cleanliness, make him a hard person to live with, but he has a kind streak that, in the words of Leonard "makes you want to, you know, not kill him". Fans of the show are split between those who want to see him become more socially aware and form more relationships, and those who find his abrupt and sarcastic manner refreshing. With the fourth season still playing out, fans of the show will have to wait and see which way he'll turn out.
"I don't like the Olive Garden. They treat me like family."
"You're far too short to be Darth Vader. You're much more likely to be a turncoat Ewok."
"They were clever, Leonard. They exploited my complete lack of interest in what you were doing."
"No, we don't. We have to take in nourishment, expel waste, and inhale enough oxygen to keep our cells from dying. Everything else is optional"