Schemer is a passive rat that inhabits the Dark Brotherhood Sanctuary in Cheydinhal. He is counted as a guild member, so other members can be expelled for killing him. However, during the "The Purification" quest, Schemer can be slain without consequence, though doing so accomplishes little to nothing as he is not a target of the quest.
Schemer has the same appearance of any other rat found in Cyrodiil. The only difference is that it bears a unique name.