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

Paul Kelly is a New York gang leader during the time the male-prostitutes are murdered. Each time a new body is found, he instigates mobs around the crime scene, in an attempt to upset the police and to encourage Kreizler to continue his investigation. In the end, Kelly sends one of his men to Kreizler's aid during the final confrontation because he knows Kreizler is the future of police investigation. Kelly likes change, especially when that change that embarrasses the power in New York City.