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: I'm done enabling him. Like, this is HIS spot. And the thermostat has to be set to his comfort level even though he doesn't even live here anymore and I'm always chilly.
Raj: Is that why you wear a hoodie all the time?
Leonard: YES.

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?

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