The Big Bang Theory: 10 Characters You Totally Forgot Existed

10. Sebastian (Sheldon's Old Roommate)

Season 2, Episode 22: The Staircase Implementation

It's easy to forget about Sebastian since he only popped up for ten seconds, his occupation wasn't given, nor was his name mentioned outside the credits. He appears so briefly, most viewers didn't notice he's played by a pre-fame Steven Yeun.

Despite Sebastian's sojourn on the show, he's important for one simple reason - he was Sheldon's roommate before Leonard moved in. As Leonard enters the apartment building, he bumps into Sebastian and asks him if he knows which floor Sheldon lives on. Before dashing outside, Sebastian implores Leonard to "run away, dude." Because of Sheldon's endless rules, stipulations and regulations, you naturally assume that living with him was too much for Sebastian to bear.

But when Leonard checks out his new bedroom, he's horrified to see "DIE SHELDON DIE" written on the wall in blood-coloured paint. Sheldon has driven all of his friends up the wall with his antics but never to this breaking point. As a result, one can only wonder what Sheldon could've done to make poor Sebastian snap like this.