ADVENT.exe, aka Advent, is the soul of a teenager fused with a demon, with the teenager in full control of the demon powers.

Adam, 16, wanted to make his own Sonic Adventure mod where they're human instead. But then, a literal demon (named Ventor) decided it was his unlucky day, went "lol sike" and made his game a Sonic.EXE clone, so Adam was like "welp, I'm just gonna publish it as a cool EXE game". Before he could do that, however, he died in his sleep (the demon managed to do it) and he has now merged souls with Ventor and Become His Own Human Sonic Dot EXE. Now he does the requisite Sonic.EXE things, but he doesn't torture his victims because he's nicer than that, for some reason. He (still killing them) just takes them into his game and makes them human versions of "fellow" Sonic characters. He wants to make his game fun scary, not just scary-scary.

Advent takes the place of Majin Sonic in Endless. His voice is essentially ~The Quintessential Sonic~, whatever that means to you.