The Voice is a villain from the Kid Chameleon series of stories. It is literally just a voice that seems to control the world of Wildside and conjures up the levels and monsters found there. Casey originally thought that The Voice was helping him through the game in order to survive it, but later discovered that Brad was taking orders from it in working at the Islecatraz prison.
After Casey's second game in Wildside, The Voice refused to let Casey leave, calling him the "ultimate challenge". Casey declared himself ready, presumably planning on taking The Voice on as a "final boss". We may never know, as the series was cancelled on that cliffhanger.