Secondary characters making a move

I've found to be true what other writers have said about secondary characters emerging as a main character. I have one and he's nothing but trouble.

Christopher is the hero of my NaNoWriMo novel this month. Yet, one of his friends, Zero, seems to be telling me he would like his own novel when I finish this one. He keeps recurring in this story almost as much as Christopher and Penelope, the heroine. 

I don't know much about Zero right now. He's just listed on my character sheet as a wealthy computer hacker. He'll have black hair and blue eyes, tall (6'2) and left handed.

I'm off to crank out another 2,500 words tonight.