William Joseph "Billy Blaze" Blazkowicz II, a.k.a. Commander Keen (or sometimes just Keen) is the hero in a series of games with the same name. Even though Keen is only eight years old, he's highly intelligent for his age; it's said that he has a IQ of 314. Commander Keen usually wears a purple t-shirt, jeans, and an American football helmet painted in Green Bay Packers colors that he borrowed from his brother.
Arthur changed his surname from Blazkowicz to Blaze to make it sound more showbiz-like.
Keen also has an older brother and a younger sister, but their names are unknown.