Sarah is a Shinon settler, and unusually quiet and shy. The only person she appears to be comfortable around is Thomas. Playing as her provides possibilities for a quite different ending scenario. You can even let her out of the combat and then have the rest of the party fight the final boss in Commander mode.
Given that Sarah and Boy are both Destined children suggests that two children survive the Death Eclipse instead of one. During the two previous eclipses 300 and 600 years prior to the game's events, one of those children were left to die in the Abyss while the other lived on.