Billy is a tiny green alien who works for "The Center", as M.O.M.'s personal secretary, often seen flying on his mini hovercraft. He is a powerful idolizer of Martin and in one episode, it was revealed that he even has a picture of Martin (making him the "crazed fan of the show" archetype.)
Usually he is first seen greeting the gang, once they are signed in and often does his signature high five with Martin. His role is generally a supportive one; he pops in sometimes during a mission to give Martin, Diana, and Java valuable information, usually startling them in the process. He also analyzes on slime or fabric samples sent by Martin during a mission. However, he won't help Martin and the gang on a mission directly unless he is forced to.
Martin and Billy are best friend's. They even have a secret handshake. Billy beyond that also really idolizes him. He is always taking Martin's advice, goes to Martin first when he has questions, he even goes as far as dressing up as him for Halloween.
Role in the series
In the two-part episode of the second season, "They Came from Outer Space", Billy was revealed to have once been a great alien warrior, known as Ganthar, on another planet, but he gave up violence for good. When Martin asked why a planet on Billy's list of hologram worlds was blue, it was revealed by M.O.M. to be the world which changed Ganthar into Billy, which is the reason he doesn't go on missions with the other three agents. He arrived on Earth in the Roswell UFO Incident in 1947.
Billy recently adopted a human disguise—the "B.S. 1000"—to be with Martin and Diana at their school. A few episodes in season 3 hinted that he liked Diana, such as in one of the Christmas episodes he invited her under the mistletoe and actually went on a friendly date with her. he seems to like Diana in some other seasons and episodes. It is revealed in the episode "Revenge of the Doppelganger" that Billy was once a super model on his home planet. This might be true, since he was powerful and might have been popular.