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: Hi. I'm Leonard. You are beautiful. You pop, sparkle and buzz electric. I'm going to pick you up at eight, show you a night you will never forget.
Raj: Where are we going?

Sheldon: I can't seem to get in touch with Amy. I tried e-mail, video chat, tweeting her, posting on her Facebook wall,texting her, nothing.
Leonard: Did you try calling her on the telephone?
Sheldon: The telephone. You know, Leonard, in your own simple way, you may be the wisest of us all.

TBBT Quotes

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

Penny: Here's a question-- as an alien pretending to be human, are you planning to engage in any post-prom mating rituals with Amy?
Sheldon: There are post-prom mating rituals?
Penny: Not always. Unless your date drives a van with an air mattress, then always.
Sheldon: Well, if it's part of the prom experience, then I'm open to it.
Penny: You're kidding.
Sheldon: I may be an alien, but I have urges.If Amy wants to copulate by firing her eggs into space, well, then, I will happily catch them with the reproductive sac on my upper flermin. I'm not the best at reading facial cues, but I can see that you're a little turned on.