Willy, known as Kabu, acts as Sally's magical assistant from the witch world Astoria. He typically appears as a five-year-old human boy and masquerades as Sally's younger brother to preserve their cover among humans. His core duties involve supporting Sally during magical tasks, aiding in problem-solving, and helping conceal her supernatural origins from human acquaintances.

Kabu possesses shape-shifting abilities extending beyond his default form, though specific alternative appearances remain unspecified. His magical capabilities sometimes falter or prove imperfect compared to Sally's powers. When Sally faces punishment for magic misuse, Kabu shares the consequences, including temporary loss of magical abilities that forces both to manage situations without supernatural aid.

Kabu's background reveals he is actually a Fallen Prince from the destroyed Gold and Silver Kingdom. Entrusted to Sally's parents as a newborn, he was raised under their protection in Astoria. This royal lineage forms part of his concealed history while living with Sally.

Character dynamics shift when Poron, another young witch, joins the group later in the original series. Kabu adapts to this trio dynamic, distinct from his initial partnership with Sally alone. His interactions with Sally gradually develop familial qualities, echoing their posed sibling relationship.

In the sequel series, Kabu continues as Sally's companion upon her return to Earth years later. He is joined by new companions including Poron and Dabdab, a magical realm cat. This expanded group undertakes fresh magical adventures while maintaining the premise of hiding their supernatural nature from human society.