Pop is Will’s father reincarnated.
Will started hearing Pop talk after his father died. Pop’s always been there to provide Will with encouragement and comfort like a father does. There’s also the bonus segment in the second playthrough, which confirms Pop protected Will from Robert when he started getting aggressive. His father might have come back because he knew Will needed him for a little longer. The Hanged Man is Will's crooked man.
All the misery in Will's life basically began with the meeting with the hanged man. He began seeing imaginary people, then his dad died, then his mom got a call from Will's "cousin" after which Will learned, what kind of scum his father was and more importantly, got the "demon child accusation" letter, which made him believe, he's the reason why David's family was ruined. The Hanged Man himself tells Will about everything, which drove him to take his life and downright tells Will, his life is steering in the same course. Isn't this actually a parallel to David's relation to Duke McGahan?