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: Why on earth are you telling me all this?
Leonard: Sometimes you movements are so lifelike I forget you're not a real boy.

TBBT Quotes

I won't say that all senior citizens who can't master technology should be publicly flogged, but if we made an example of one or two, it might give the others incentive to try harder.

Sheldon

Sheldon: Why on earth are you telling me all this?
Leonard: Sometimes you movements are so lifelike I forget you're not a real boy.