The Joker (Telltale)

"John Doe" is a patient at Arkham Asylum. No one knows about who he is and what he is doing at the Asylum, including the staff themselves.

Bruce Wayne
John was very welcoming to Bruce when he first arrived, personally saving the billionaire from a group of patients that tried to attack him. He even willingly organized him to give him a chance of making a phone call. Dialogue he shares with Bruce hints that he may know the billionaire is actually Batman.