Leonard HofstadterLeonard is Sheldon’s roommate. A highly intelligent experimental physicist, he’s often looked down upon by Sheldon for not being a theoretical physicist. Leonard has learned to live with Sheldon’s many eccentricities, and they have developed a strong bond with each other.
Leonard received his Ph. D at the age of twenty-four, and now works at the same university as Sheldon, Rajesh and Howard. His work often receives comical derision for being derivative, something he has been mocked for his entire life due to a distant mother.
Leonard’s mother displays many of the same traits as Sheldon, and it is possible that this is a reason for Leonard’s ability to tolerate Sheldon.
The writers immediately hinted to a possible romance between Leonard and Penny, and their feelings for each other has been a major source of comedy for the show, much like the Ross & Rachel situation from the sitcom Friends.
Leonard is considered the “straight man” of the show, and helps guide the viewer into the eccentric world of geniuses.