Who's the best friend of Sheldon Cooper? --- Leonard !

Leonard Leakey Hofstadter Ph.D., is a fictional character on the CBS television series The Big Bang Theory, portrayed by actor Johnny Galecki.

Leonard Leakey Hofstadter, Ph.D., is an experimental physicist at the California Institute of Technology who rooms with Sheldon Cooper in apartment 4A of their apartment building. Originally from New Jersey, Leonard attended Princeton University and was twenty-four years old when he received his doctorate with a Dissertation of the Year award for experimental particle physics. Having lived with Sheldon for ten years as of Season 6, he and Leonard are best friends.

However, Leonard can become very agitated by many of his eccentricities and often feels obligated to explain Sheldon's behavior and serve as a mediator between him and other people in situations – he has been described as the straight man of the series.

While Leonard, along with his geek friends and fellow scientists, is somewhat socially inept, Leonard would like to be better accepted outside of his social group. Leonard is more well-adjusted than his companions, being more willing to try different things. He appears to be best suited as a leader of the group, with Sheldon commenting how Leonard is the "Kirk" of the group. The writers have toyed with a romance between him and neighbor Penny, with their on-and-off relationship being a major force for drama.