Sophia Myles as Mercedes "Mercy" Sullivan

The Angels' resident medium and their true connection to the occult world and the Powers That Be bravely joins Taryn and James on their apparent patrol armed with the fearsome -- ah -- all right, flashlight. She didn't seem quite appropriate with any weapons, and I didn't want to use another book animation or anything. She's probably helping them look for some lost artifact or key to some puzzle or something.