Pepsi in monster form (BACKGROUND), above SMG3, with SMG4 facing away from the camera.
Pepsi is the (terribly named) adopted demon monster son of Mario, and is intent on both ruling the world and trying to make friends. Just a total *sshat if you don't know him well enough to say otherwise.
He hates his "father" because Mario is an idiot and just named him the first thing that came into his head.