Sarah is an actress from Marble Hornets. Her character in the film, also named Sarah, is friends with Tim and is the ex-girlfriend of Brian after he left town. She first appears on-screen in Entry 9.
Sarah has not really been seen out of acting mode, but based of her appearance in her entry on camera she is a kind person who defends Seth, and had become fed up with Alex Kralie's change of behavior.
Appearances in Series
Warning: If you have not yet watched the entry yet do not watch the appearance entry description
Sarah is also speculated to be the Hooded Man (or in this case Woman) although there is not a lot of evidence pushing towards it. Entry 84 and Entry 86 deny this theory.
Some believe that Sarah is Totheark and that is explained in the following video: