Steven Yeun (Born "Yeon Sang-yeop") is a Korean born American actor, best known for playing the character Glenn Rhee on AMC's hit series The Walking Dead, which portrays the struggle of a group of survivors following a sudden zombie apocalypse. Prior to The Walking Dead, Yeun had several acting roles, which included a minor appearance in a single episode of the Big Bang Theory, where he played Sheldon's former roommate "Sebastian" in the Season 3 flashback episode "The Staircase Implementation". Yeun was best known for being a series regular on "The Walking Dead" until his character was killed off in season 7.
Yeun only appears for a moment in the episode "The Staircase Implementation". His name is not stated, but was given in the credits as "Sebastian". Sebastian is the first character from the Building of whom Leonard meets, and warns Leonard to "Run away, dude", when Leonard goes to the apartment building in response to an ad for a roommate placed by Sheldon. Sebastian's intolerance toward Sheldon's behavior becomes apparent when Leonard enters Sebastian's former room to the phrase "DIE SHELDON, DIE" written on the wall in red paint. Sebastian was suggested to have had a scientific background of some sort, being that it was Sheldon's standard for a roommate. It is unknown if Sebastian and Sheldon had gotten the apartment together, or if he had come in before or after Sheldon had moved there.