Sebastian

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", when Leonard tells Penny the story of how he first met Sheldon back in 2003.

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 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 checks out the bedroom, the words "Die Sheldon Die" are written on the wall in red paint, which were written by Sebastian. Sheldon comments that Leonard might want to redecorate.