Who is Sheldon's best friend and roommate?

Sheldon’s best friend and roommate is Leonard Hofstadter. Leonard is an experimental physicist at Caltech who shares an apartment with Sheldon Cooper when The Big Bang Theory begins.

Their relationship is deliberately complicated. Leonard is patient, socially capable, and often acts as a mediator when Sheldon’s rigid habits or blunt remarks create conflict. Sheldon frequently belittles Leonard’s work and behavior, yet he also recognizes Leonard as his closest friend. Their long history allows Leonard to understand Sheldon’s routines and unusual rules better than almost anyone else.

Leonard’s role as Sheldon’s roommate also drives the show’s central relationships. Penny moves into the apartment across the hall, and Leonard’s attraction to her becomes one of the series’ major storylines. Raj Koothrappali and Howard Wolowitz are also among Sheldon’s close friends, but neither is his roommate in the show’s opening setup. Stuart Bloom is a comic-book-store owner and occasional member of the group, not Sheldon’s best friend or regular roommate.