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

It's very simple. Scissors cuts paper, paper covers rock, rock crushes lizard, lizard poisons Spock, Spock smashes scissors, scissors decapitates lizard, lizard eats paper, paper disproves Spock, Spock vaporizes rock, and, as it always has, rock crushes scissors

Sheldon

Penny: Hey, Sheldon, did you change your Wi-Fi password again?
Sheldon: Yes, it's "Penny, get your own Wi-Fi." No spaces.

TBBT Quotes

James Earl Jones: Why don't you and your friends come to Comic-Con with me?
Sheldon: Really?
James Earl Jones: Of course. And San Diego is right across the border from my favorite city on Earth, Tijuana.... where I'm taking you every night!
Sheldon: Ay-yi-yi.
James Earl Jones: Ay-yi-yi. Bang. Bang.

Sheldon: Why do you have the Chinese character for "soup" tattooed on your right buttock?
Penny: It's not "soup," it's "courage."
Sheldon: No it isn't. But I suppose it does take courage to demonstrate that kind of commitment to soup.
Penny: How'd you see it? You said you wouldn't look.
Sheldon: Sorry. As I told you, the hero always peeks.