Yin is a "Doll", a type of medium that came into existence with the appearance of Hell's Gate. Her kind has an affinity for certain substances or elements which gives them a special ability. When they are in contact with that substance, it allows them to project “observation specters” to any location the substance is also present, and lets them view that location remotely. Yin’s medium substance is water.
Beautiful cosplay by Fantasia of The Lost Aria Group from New Zealand! They have an impressive collection of photos in their Deviant Art account, so please pay them a visit! Thanks again to the group for allowing me to post this!