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

Sheldon: You have to check your messages, Leonard! Leaving a message is one-half of a social contract, which is completed by the checking of the message. If that contract breaks down, then all social contracts break down and we descend into anarchy.
Leonard: It must be hell inside your head.
Sheldon: At times.

Look, you know, words don't always have to mean things.

Leonard

TBBT 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?

Raj: Okay, here's another one: if a zombie bites a vampire, and the vampire bites a human, does the human become a vampire or a zombie? Or, a zompire?