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.
Some believe that Sarah is Totheark and that is explained in the following video: