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 recently read that during World War Two, Joseph Stalin had a research program to create supersoldiers by having women impregnated by gorillas.
Howard: What a sick use of science.
Raj: Hey, as long as the baby's healthy.
Amy: I wonder if Stalin considered any other animals.
Leonard: Hippos are the deadliest creature. A half-human, half-hippo soldier would be pretty badass.
Howard: Yes, but when they're hungry-hungry, you can stop them with marbles.
Sheldon: Yeah, the correct animal for interspecies supersolider is koala. You would wind up with an army so cute it couldn't be attacked.

TBBT Quotes

Penny, there's only one cookie with something in the middle that solves life's problems, and that's an Oreo. Or a Nutter Butter, if you're in a pinch.

Sheldon

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