You go into investigation at the old Mario's Mania Playhouse. The animatronics are decommissioned and covered in blood. Mike Robertson AKA The Costume is the prime suspect. You couldn't find him though. Until 3 days later, when you went back to the place on your own and found him standing motionlessly. But he wasn't a statue, neither was he even human. He can still speak, but he became trapped in the suit for so long that he grew into it. His vocal chords got changed to a point. Where he sort of sounds like Mario. But his voice was also dark and grimy. Very, very deep voice. But he also doesn't have Mario's power-ups. Because of course, it is just a costume. You thought to yourself. "Of course the costume had to be eyeless.", But Michael can see just fine

Costume: "The funny thing is that I never actually left. As you can see, I am now grayscale. But, the real question is, can you survive? I've killed 6 kids and I'm not afraid to talk about it. I am the Costume, this costume is me. Let's settle this 1 on 1."