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: Leonard, you're my best friend. Why don't you ever take my side.
Leonard: Because I can never understand your side!

TBBT Quotes

Sheldon: Why are you crying?
Penny: Because I'm stupid!
Sheldon: Well that's no reason to cry; one cries because one is sad. For example, I cry because others are stupid and it makes me sad.

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