Dr. Leonard Hofstadter, a physicist at Caltech, got his PhD at the age of 24 thanks to his 173 IQ.  Leonard lives with his best friend and co-worker Sheldon.

Leonard is definitely the most social of his nerd group of friends and has a crush on their neighbor, Penny.  Leonard went on a date wih Penny at the end of season one, but it didn't work out due to their insecurities.

During the third season of Big Bang, Leonard returned from his artic expedition with the guys and began dating Penny.

Leonard Hofstadter Quotes

Leonard: So, is that it? Are we engaged?
Penny: Yeah, I think so.
Leonard: All right.

That was Wil -- he's feeling a lot better. Apparently, he's 12-down in the TV Guide crossword puzzle.

Leonard
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TBBT 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

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.