Children’s book about a kid and an alien represented by an asterisk (*)

I read a book as a child, probably in the early 1990s, whose story sick with me but which I haven’t been able to find since. The plot goes thusly: a child plays a semi text-based video game in which he controls a character (represented by an asterisk), but it later becomes apparent that the character he’s controlling is real and self-aware. He’s an alien, or something similar, and he needs help… and that’s most of what I remember, but I can’t be the only one who read it, right?