1. He's the most important representative of FNaS: FF "II!" (and FNaS: FF "EE!?") overall a titular/main Antagonist and the Main Mascot of Fierce Forces in
Sonic's Heroes Tera Pizza Promenade/Sonic Segablur's Prime Pizza.

Sonic "Tune-Up!" likes to stalk Silent a lot and thinks he's the best.
He appears as well in the comic addon as the start point and his genesis installment. Event before main story mode, in FNaS1: Insane Immerse.

2. Just a slightly injured Modern Clone Sonic with some bandages wrapped
on his hands and legs.

He likes to lift gym weights and stretch his muscles.
Appears as well in the comic addon, in FNaS2: Insane Immerse.

3. He's also one of important representative of FNaS: FF "II!" (and NFaS: FF "H!&H!") and known as the: Destroyed Clone. I don't think there's a need to say anything more, but Silent is afraid of him the most. Plus... this guy has two hands in this version.
Not One... But Two! Yes, one is a skeleton hand! And that makes him more dangerous.

Did I mention that the Protagonist is scared of him the most from all the characters... ok I said that. Next! Appears as well in the comic addon, in FNaS3: Insane Immerse.

4. A darker looking vampiric version of Sonic who likes to scare and feed on Silent's F.E.A.R!
Silent is the reason, why Nightmare Sonic "Dire-Up!" now hunts her, instead of
Luigi "Superb-Up!" in Fierce Forces.

Because he became hers own literal Nightmare, due to Silent's sacriface of taking
the characters as her own realistic imaginations in the new game rule.
Appears as well in the comic addon, in FNaS4: Insane Immerse.

5. A total massacre... worse looking appearance, than Salvage Sonic "Gory-Up!".
He still has two hands. Also he's a ghoul.

Even in this state, Silent is still afraid of the Clone from the previous 3rd installment appearance, despite of Nightmare having the name Salvage. Because back then, when he was seriously injured, despite that, he was still... Alive!
Appears as well in the comic addon, in FNaS4 DLC: Insane Immerse.