Who is Paul Kelly from The Alienist and what is their importance?

Paul Kelly was a New York gang leader during the boy-prostitute murders. He makes sure that he instigates the mobs around the crime scenes every time a body is found as encouragement for Kreizler to continue his investigation. Kelly even sends one of his men to Kreizler's aid in the final confrontation because he knows Kreizler is the future of police investigation and he wants to be able to say he was involved from the ground floor up. Kelly embraces change, especially when that change causes embarrassment to those in power.