Jambu is an orca whale (killer whale) who performed tricks at Denver Sea Park. In "Free Willzyx", the boys and other members of their class are tricked by the Jambu show's announcers into believing that Jambu is actually an extraterrestrial named Willzyx and once lived on the moon. Having believed them, the children race to take Jambu to Clyde Donovan's house, where he has a pool, and then to Mexico where the whale was to be sent to the moon in a rocket. Jambu died after being sent into space and its body now lies on the moon.
Jambu made a brief cameo, still dead, when Tom Cruise was sent to the moon in a similar fashion. Both lie next to each other on the moon.
Jambu looks like a typical killer whale. It is mostly black with a white underside and red inside its mouth.