Dark Sonic, a Sonic from a different reality who has embraced his dark form and was consumed by its power. He can match up to Nathan's and Blade's Super 2 form, but he does have a transformation called Dark Rage Sonic.

This is his Dark Rage Form.

Sonic.EXE is a Sonic who's body is possessed by a demon called Exetior. He was summoned to this world by ??? to help destroy this world version of Mobius. He can go Super, which makes him insanely powerful, but he has a form called Devil Blood Exe.

Then we have a surprised villain, Avery's lost long brother, Shin. He willingly joined ??? in  destroying mobius, he doesn't care if he has to face his sister, he would do anything to destroy the world where his mother grew up.

Now we have Slade The Hedgehog, an evil version of Blade who is hell-bent on world domination. After being stopped multiple times, he was summoned by Omen to rule over this world, making him smile at this.

And finally, we have the main villain himself, Omen. His power is unlike anything the freedom fighters had dealt with before. Knuckles knows that Omen exists because he saw a vision of it in the Master Emerald. But as of right now, Nathan and the others don't know who he is.