Fifteen is young (though probably by now a bit older than his nickname). He usually dresses in shorts and an ancient T-shirt (depending on the weather), and his head is usually shaved.
He does not appear in A Sane Woman.
We first see him in the fourth chapter, "Prove It (Just the Facts)." He comes into The Quarter and describes the shootout that happened in Chapter Two, "A Quiet Night at Duffy's" (which he witnessed from a safe distance).
In Chapter Seven, "starling," we find out that he has (unsuccessfully) been trying to get Christy from the Jinx to go out with him.
He appears many more times in the novel, mostly as a roadie for the band Kingdom Come, and sometimes as a runner. In Chapter Nine, "Curse the Darkness," we find out that he's T.C.'s nephew (or, as she puts it, niece). He is Pete's friend, and Pete finds out in Chapter Thirteen, "The Forces at Work (Part One)" just how good a friend he is.
the mystery stories
the third novel
Fifteen's first appearance in the third novel is in the chapter "On the Medical Team," where he appears in his new function as an all-purpose aide to Doc and the others.
In the next chapter, "Quartet," we find out that he is still friends with Pete and Katherine, helping Pete while he is laid up.
In "Distance and Time," SarahBeth expresses some disbelief about Fifteen and Christy, and Katherine reveals some history which was implied but never explained in detail.