Sleuth is Simon's Tracker Ball. It used to be a bald old tennis ball until Simon picked it up and incubated it, which
apparently makes Simon dangerous because Aunt Zelda says: "If he's incubated a Tracker Ball, than who knows what else he's capable of?" A Tracker Ball is a Darke object. At one point, Simon told it to put a Tag on Jenna (that is, follow her around) until Simon reformed and Removed Sleuth's Darkenesse also removing the tag on Jenna. Sleuth can melt down and become a pile of melted rubber but to do this it must heat up first, therefore enabling it to burn somebody or something. When it does this, it glows green.