|The Big Bang Theory Staff|
|Birthday||December 21st, 1983|
|First Episode||The Staircase Implementation|
|Number of Episodes||1|
|All of the Big Bang Theory Staff|
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 add 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.