How about a Schizophrenic? They pick one player to 'target' each month, and will randomly protect that player, inspect that player, kill that player, or do nothing.
Also, if there's more than one cop/investigator, one could be insane (gets innocent results on Mafia and guilty results on town, but can still be helpful if he can realize this (I realize that due to the Salmoneous mixup, I may have been insane this round)).