Silver Sarah is a white woman in a silver dress and high heels with brown hair and wearing sunglasses. She is basically a silver recolor of Meesterious Mia.
Silver Sarah can only be used in Serious Sam HD: The Second Encounter if the player has the Serious Eight DLC. Otherwise, she cannot be used. Since the Serious Eight DLC is currently impossible to obtain, those who don't have the DLC won't be able to play as her for a long time, if not ever.