The first name of Leonard Hofstadter's roommate in The Big Bang Theory is Sheldon. Sheldon Cooper shares Leonard's Pasadena apartment when the series begins, establishing the roommate relationship at the center of many early storylines.
Leonard and Sheldon are both Caltech physicists, but their personalities could hardly be more different. Leonard is generally accommodating and socially aware, while Sheldon is rigid about routines, schedules, cleanliness, temperature, and household rules. Their roommate agreement turns everyday matters into elaborate negotiations, giving the show many of its recurring comic situations.
Sheldon is played by Jim Parsons, while Leonard is played by Johnny Galecki. Penny lives across the hall and becomes Leonard's main romantic interest, so she is a neighbor rather than his roommate. Rajesh Koothrappali and Howard Wolowitz are Leonard's close friends and colleagues, but neither is his regular apartment-mate. Sheldon and Leonard remain closely connected even after their living arrangements change later in the series.