This is Death, aka Grim Sonic. He isn't an EXE, but he is involved with the EXE verse. He's a being that takes on the form of Sonic so that those who play his version of Sonic 3, a game that has been modernized to fit the person's needs, basically is like a personal experience to those who play it. Sonic characters reenact their memories, fears, secrets, and even desires before the victim's inevitable death arrives once they have finished the game completely. He chooses whether or not your time is up truly and he picks his victims carefully. Once he has picked his victim, the impending doom is near. There is no escaping Death.