Writing my first YA novel

A character named Vinnie just popped onto the page. Who is he? How does he know my main character? And most of all, what is his purpose? According to Deborah Halverson (@Dear_Editor) in Writing Young Adult Fiction for Dummies, “If a new character happens to pop into your story as you’re writing it, let him hang out a while to see whether he fits in. Secondary characters have a way of arriving before you even knew you needed them…”  Okay, Vinnie. Show me what you’ve got!