Dr. Sheldon Cooper is Leonard's best friend, roommate, and co-worker at Calech.  Sheldon is the smarter of the two with an IQ of 187, but is definitely the more socially awkward of two.  He has a ridiculous ego, is extremely stubborn, but the group seems to put up with him.

Sheldon Cooper Quotes

They can't just cancel a show like Alphas. You know? They have to help the viewers let go. Firefly did a movie to wrap things up. Buffy the Vampire Slayer continued on as a comic book. Heroes gradually lowered the quality season by season till we were grateful it ended.

Sheldon

Penny: I'm a Sagittarius, which probably tells you way more than you need to know.
Sheldon: Yes, it tells us that you participate in the mass cultural delusion that the sun's apparent position relative to arbitrarily defined constellations at the time of your birth somehow affects your personality.
Penny: Participate in the what?

TBBT Quotes

They can't just cancel a show like Alphas. You know? They have to help the viewers let go. Firefly did a movie to wrap things up. Buffy the Vampire Slayer continued on as a comic book. Heroes gradually lowered the quality season by season till we were grateful it ended.

Sheldon

I apologize if I haven't been the friend you deserve. But I want you to know, in my way, I love you all.

Sheldon