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.