Thanks to Imagination and Memorization of Silent "Player-On!" once she accepted the Game Rules of Alter Dr. Golden.
(Mega Mere Max Mix (Alternate Maniac Mania) after this event, has changed into Fierce Forces).

The Characters changed into the modern "leveled up" variations (Origin Sonic "Midday-Up!", Plush Origin "Morncloud-Up!", Ragdoll Origin "Tranquil Taciturn" and Classic Sonic "Genesis-Up!" are the exceptions, they are considered as the "leveled up" classic variations).

Some of the Characters, received changes to complete and make more suitable names and their "code names" as well.

The List of Characters that are and are confirmed to be in "Imagine It!" Comic series.

FNaS: II = Five Nights at Sonic's: Insane Immerse
(An Event (Addon) happening just before Fierce Forces on South Island)

FNaS: FF "II!" = Five Nights at Sonic's: Fierce Forces "Imagine It!"

Series in chronological order:
(Mega Merge Max Mix → Insane Immerse → Fierce Forces)
(Classic Era → → → → → → → → → Modern Era)

Please Note that some Characters are hidden / not mentioned at all in this list for No spoiling purpose.

Cast of FNaS 1: II and in FNaS: FF "II!"

Clone Sonic "Tune-Up!"
Clone Mario "Tune-Up!"
Clone Yoshi "Tune-Up!"
Clone Tails "Tune-Up!"
'15 Golden Sonic "Memorize-Up!" / Golden "I"
????? ?????? "????-??!"
????? ??? "????-??!"

Knuckles "Recall-Up!" / Security Guard Knuckles

Cast of FNaS 2: II and in FNaS: FF "II!"

Mechano Toy Sonic
Mechano Toy Mario
Mechano Toy Yoshi
Mechano Taingle
Mechano Balloon Toad
Mechano Luiginette
Withered Sonic "Sore-Up!"
Withered Mario "Sore-Up!"
Withered Yoshi "Sore-Up!"
Withered Tails "Sore-Up!"
'17 Golden Sonic "Memorize-Up!" / Golden "II"
Mechano Toy Tails
Tails Doll "Stitch-Up!"
Paper Plates Duo - Paper Sonic "Reshape-Up!" & Paper Mario "Reshape-Up!"
The Shadow Triple - Shadow Mario "Dusk-Up!" & Shadow Yoshi "Dusk-Up!" & Shadow Tails "Dusk-Up!"

Amy Rose "Remind-Up!" / Security Guard Amy

Cast of FNaS 3: II and in FNaS: FF "II!"

Salvage Sonic "Gory-Up!"
Phantasm Mario "Jeer-Up!"
Phantasm Yoshi "Jeer-Up!"
Phantasm Tails "Jeer-Up!"
'27 Golden Sonic "Memorize-Up!" / Golden "IV"
Withered Mechano Toy Sonic
Withered Mechano Toy Mario
Withered Mechano Toy Yoshi
Phantasm Mechano Taingle
Phantasm Mechano Balloon Toad
Phantasm Mechano Luiginette

Modern Sonic "Reversion-Up!" / Sentinel Martial Artist Sonic
Silver "Reversion-Up!" / Sentinel Martial Artist Silver

Cast of FNaS 4: II and in FNaS: FF "II!"

Nightmare Sonic "Dire-Up!"
Nightmare Mario "Dire-Up!"
Nightmare Yoshi "Dire-Up!"
Nightmare Tails "Dire-Up!"
Nightmare Golden Sonic "Memorize-Up!" / Golden "V"
Mechanight Toymare - aka. The Mergefright
Phantasm Salvage Sonic "Fleer-Up!"
Phantasm Amy "Remainder-Up!"
Night Terror Sonic - aka. Insomnic
Plushnic & Plushtails "Hum-Up!"

Luigi "Return-Up!" / Luigi "Superb-Up!" & Security Guard Luigi

(depends on which job is chosen for Luigi by Silent, either being on roaming mode all the time or being as the invincible comrade avaiting to be summoned for a limited time in Fierce Forces)

Cast of FNaS 4 DLC: II and in FNaS: FF "II!"

Nightmare Mechano Toy Sonic
Nightmare Mechano Toy Mario
Nightmare Mechano Toy Yoshi
Nightmare Mechano Taingle
Nightmare Mechano Golden Toy Sonic
Nightmare Salvage Sonic "Creepy-Up!"
Nightmare Mechano Luiginette
Lunightterror (A manifestation created in Silent's mind by Insomnic)
Plushrio & Plushi "Boo-Up!"

Silent "Avatar-On!" (She deals against her nightmarish visions, instead of Luigi "Return-Up!")

Cast of FNaS 5: II and in FNaS: FF "II!"

Origin Sonic "Midday-Up!" / Golden "α" (Alpha)
'?? ?????? ????? "????????-??!" / Golden "∅" (Null)
'15 Golden Sonic "Memorize-Up!" / Golden "I"
'17 Golden Sonic "Memorize-Up!" / Golden "II"
'22 Golden Sonic "Memorize-Up!" / Golden "III"
'27 Golden Sonic "Memorize-Up!" / Golden "IV"
Nightmare Golden Sonic "Memorize-Up!" / Golden "V"
Shadow Sonic "Midnight-Up!" / Golden "ω" (Omega)
(Technically he appears shortly in 1, 2, 3, 4, 6 of Insane Immerse installments, except 4 DLC)
Dr. Golden "Prepare-Up!" / Golden "∞" (Infinite) ↔ Dr. Eggman "Conquer-Up!"
(Can switch between his hedgehog / human appearances)
(And this confirms, that he appears in FNaS 5: II as well and he's the part of the Golden Team)

Plush Origin "Morncloud-Up!"

Classic Dr. Eggman "Scheme-Up!" / ???????? ????? ??????
Classic Tails "Genesis-Up!" / Security Engineer Tails
Classic Sonic "Genesis-Up!" / ????? ????? Classic Sonic

Cast of FNaS 6: II and in FNaS: FF "II!"

Floral Lefty Sonic "Rosace-Up!"
Fortunate Mechano Righty Sonic "Clover-Up!"
'22 Golden Sonic "Memorize-Up!" / Golden "III"
Mechano Golden Toy Sonic
Mechano Glitter Toadette
Ominous Tails "Awe-Up!"
Nightmare Amy "Vile-Up!"
Mechano Sonny

Cast of FNaS: FF "II!"

Dr. Golden "Prepare-Up!" / Golden "∞" (Infinite) ↔ Dr. Eggman "Conquer-Up!"
DJ Stellar Espio
Top Stellar Shadow
Imaginal Infinite
Leo Metal Sonic
Dance Martial Artist Surge
Counterfeit X (Fake X (Mega Man))
Abyss Crystalline King Boo (He can summon his servants: Abyss Crystal Boo)
????? ?????? "????-??!" (Same Character from FNaS 1: II)
????? ??? "????-??!" (Same Character from FNaS 1: II)
'?? ?????? ????? "????????-??!" / Golden "∅" (Null)
????? ?????
Glitchnic "Hack-Up!"
Clone Sonk :) "Smile-Up!"
Plush Origin "Morncloud-Up!" (FNaS 5: II - later, in Fierce Forces returns as Silent's plush companion)
The Bomberman Team ~ Bomberman "Whiteness-Up!" & Bomberman "Blackness-Up!"
(The 1st one is sided with Silent. The 2nd one is sided with Dr. Golden "Prepare-Up!")
Zeo Metal Zee
Adventure Pac-Man
Giana "Twin-Up!"
Fiery Giga Bowser
Phantasm Luigi "Sneer-Up!"
(Not a real spirit, but an entity made from mist and he's on Silent side)
Spectrum Cyan "Aftermath-Up!"

Silent "Avatar-On!" & Silent "Player-On!" / Security Guard Silent & Soldier Grandeur Silent
Shadow "Rely-On!" / Specialite Agent Shadow

Characters that are in Fierce Forces series, but only appears in the fangames:

Nocturnal Frights at Sonic's: Fierce Forces "Hush! & Hustle!"

Phantasm Modern Sonic "Residue-Up!"
Ragdoll Origin "Tranquil Taciturn"
Ripplush Salvage "Sonorous Shriek"