Who is the man in the elevator and how does William know him? From the book balled Long way down book?

The man in the elevator is Buck, who is a friend of William's brother, Shawn. William knows him because Buck was the one who gave Shawn the gun that led to his death. Buck also tells William that he knows the rules of the neighborhood and that he must follow them, including seeking revenge for Shawn's death.

How does will react to seeing him?

William is shocked and angry when he sees Buck in the elevator. He is torn between the desire for revenge for his brother's death and the consequences of following the cycle of violence that has plagued his neighborhood. William struggles with conflicting emotions and ultimately has to make a difficult decision about what he will do next.