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.
The family wrote research papers for Christmas and they were returned graded with his stocking.
While in bed one night after sex, Leonard admits to Penny for the first time that he loves her.
Penny is troubled by the potential serious turn in the relationship because she has commitment issues and can not respond in kind, and she breaks up with Leonard. Leonard begins a relationship with Raj's sister Priya. She admits that Leonard is a great guy and she should have never broken up with him.
Leonard instantly falls in love with her, even inviting her to their apartment for lunch.
The boys are shown to be friends with Howard Wolowitz and Rajesh Koothrappali.
At age eight his mother humiliated him by making him give back a ribbon at a science fair.
He said it was his first science fair and that he had to give the ribbon back because he had done "Do Lima Beans React Better to Classical Music? His brother had previously done "Do Lima Beans Grow Worse to Rock N' Roll? At age ten, he built a hugging machine to compensate for lack of affection which his father used to borrow ("The Maternal Capacitance", S2E15).
She takes the boys to get her TV from Kurt, but he steals their pants and they return home without them.
Penny apologizes for putting them through it and later she and the four guys go out for dinner (Pilot).
Penny almost breaks up with Leonard because, to her, the relationship seemed boring.