The Player is Mr Afton
This is the most popular theory in Five Nights at Freddy’s: Sister Location at the moment. Fans are theorizing that the player character is actually the Purple Guy, or William Afton. Afton’s daughter tragically passed away when she got grabbed and killed by a metallic toy (Circus Baby) that he gave to her. Years later, Afton chooses to head back to Sister Location in an attempt to come to terms with the tragic events that passed. This theory is granted credibility by the fact that we hear a lot of dialogue lines from Ennard/Baby discussing an “accident” and telling the player that they need his help, and ultimately questioning his reason for “coming here” in the first place.
In the end, Afton gets trapped by the animatronics, and his body becomes a vessel for Ennard. This also ties in very neatly with the events of the original Five Nights at Freddy’s, and explains why Afton had an urge to kill.