What is the name of Sheldon's roommate before Leonard?

Sheldon’s roommate before Leonard was Sebastian. He appears in the Season 3 episode “The Staircase Implementation,” which reveals how Leonard first moved into Sheldon’s apartment.

In the flashback, Leonard arrives to answer Sheldon’s roommate advertisement and encounters Sebastian leaving the apartment. Sebastian warns Leonard that Sheldon is “crazy,” giving Leonard an early glimpse of the difficult living arrangement he is considering. Sebastian had apparently reached his limit with Sheldon’s rigid habits and unusual behavior.

The episode, which aired in 2010, is notable because it fills in an important piece of the series’ backstory: Leonard and Sheldon did not simply become roommates at the start of the show. Leonard replaced Sebastian in 2003, before the events of the sitcom’s pilot. Steven Yeun, later known for The Walking Dead and Minari, plays Sebastian in the flashback.

A common mix-up is confusing Sebastian with later occupants or temporary roommates, such as Raj or various roommate applicants. Sebastian is specifically the named roommate who lived with Sheldon immediately before Leonard.