Sebastian was Sheldon Cooper's former roommate before Leonard Hofstadter. He was first and only seen during a flashback in the episode "The Staircase Implementation" (S03E22) when Leonard tells Penny the story of how he first met Sheldon back in 2003. Sebastian's name is only given in the credits of the episode, and not spoken in the episode.
Leonard arrives at the apartment building having just started working at the university. Sebastian is seen exiting the elevator with all of his belongings as he is moving out of the building. Leonard asks him where Sheldon Cooper's apartment is and Sebastian tells him to run away. Leonard is confused by this statement and Sebastian tells him to "Run fast, run far," as he exits the building for good.
Leonard mentions to Penny that Sebastian had tried to warn him off, but at the time, Leonard suspected Sebastian could have been the crazy one as he had a deranged look. Penny says he probably had the deranged look from having lived with Sheldon for too long which Leonard admits is probably true. Later, when Leonard checked out the bedroom, the words "DIE SHELDON DIE" were written on the wall in red paint. This graffiti was painted by Sebastian who was fed up with Sheldon. Sheldon comments that Leonard might want to re-paint.