The Mouse Thief appears in the episode "Little Brother," in which Kent approaches him for a sacred flame, one of the three elements the blacksmith needs to craft Kent's new sword. The thief tells Kent to drink from a stream of sap to become immortal. Kent dips his foot in but explains that he was born earlier that day, so he does not have a sense of his own mortality. The Mouse Thief then hands Kent the sacred flame (a firefly).
He wears dark, tight-fitting clothing resembling that of a stereotypical ninja.