Billy is one of those kids that you don't really know anything about. His point in the novel is to serve as an example of what not to do. Like the kids in the commercials who text and drive. You know what we're talking about.
Tom fears that Billy is dead for a while, and eventually the Spook uses Billy's story to teach Tom a lesson. Poor Billy cut corners when binding a boggart, and the boggart bit off his fingers and sucked out his blood. What a way to go!
Despite that trauma, when Tom sees Billy's ghost, he seems "happy and at peace" (11.62). Well, good for him. Guess he has no idea that he's thought of as nothing more than a statistic. At least he's the kind of statistic that helps Tom to stay alive.