Marvel Boy was an add on from the Runaways, I think. He had his own series and then became a love interest/foil for Karolina. Somehow he got absorbed into the Young Avengers (I barely remember the storyline but I think it was something about Hulking's royal bloodline).
The way I see it, if I want him to be Speed, then he's Speed, and no one can stop me, dangit!