The main character. A science-loving kid who builds a machine that drills holes in cheese that malfunctions, leaving him along with Zeke and Marsha stuck in the center of the Earth.
A long-haired, preteened slacker who was assigned to work with Billy. He often tries to appear cool and aloof, but in reality gets emotional quite easily.
A wannabe social butterfly and budding journalist who was assigned to work with Billy. Very impressionable and lively, if somewhat spacey at times.
Billy's pet rat who spends much of the episodes lazing around.
- Half-Dressed Cartoon Animal: Wears only a white tank top and green sandals.
- Lazy Bum
- The Stoic: He doesn't really react much to what goes on in the show. A flashback in "Rat's Entertainment!" reveals that he's Seen It All.
- Team Pet
- Your Size May Vary: He can vary from being the size of a normal mouse to half the size of Billy depending on the scene.