NOTE: Keep in mind that you have to have the right requirements to enter his lair (see article Keep out signs for what they are).
He is a scary looking rat creature and is the "boss" monster for several of the mini-games you play in your backpack, like Combo Drop and Bounce. When you defeat him, you can choose one of his rat furniture treasures as your reward and you can place it in your own home like all the henchmen do when they're defeated.
NOTE: This has to be done quickly as you cannot hang around Rat Wraith's Lair for long after winning.
Rat Wraith's real name is Glen McCheese. He became the Rat Wraith when Glen decided not to work, and to steal instead. He boarded a plane leaving Canal City which had money, and jumped off it with the money. Unfortunately, he didn't have a parachute. He landed in a well, full of Ichor which turned his corpse into an undead ghoul. It is unknown if the Rat Wraith still knows about his life as Glen McCheese.