The Classic Version: Alter Night Guard Knuckles
from Five Nights at Sonic's 1: Different Dimension

Although, he does not appear in Mega Merge Max Mix
(Because there's only Alter Night Guard Luigi, just like in og Maniac Mania)

But later, he will turn into The Modern Version: Security Guard Knuckles.

Firstly, after the event of Five Nights at Sonic's: Mega Merge Max Mix,
(ported to the modern version of the game, accepting the new game rules of
Alter Dr. Golden -> Dr. Golden "Prepare-Up!" by Silent "Player-On!")
the Modern Version of Knuckles appears in
Five Nights at Sonic's 1: Insane Immerse as: Night Guard Knuckles "Abide-Up!"
(Pizzeria on South Island - Addon Comic) /

Five Nights at Sonic's: Fierce Forces "Imagine It!".
After Insane Immerse, being "SAVED" by Silent "Avatar-On!" he gets upgraded to Security Guard Knuckles and he's promoted to Leader of the faction: Valorous Resistance.
(Arcade Machine in Dr. Golden's Prep Mean Haze Citadel on Angel Island
- Main Comic)

Security Guard Knuckles is a summonable comrade in "Imagine It!" and he's notably a prefect protector of Silent "Player-On!" against the Modern Clones, that originated from Five Nights at Sonic's 1: Insane Immerse and are now all together in
Five Nights at Sonic's: Fierce Forces.
Especially, he's good fighting against the Leader of the Band: Sonic "Tune-Up!".

Spoiler Alert: Could the Security Guard be mentioned in
Five Nights at Sonic's: Fierce Forces "Entertain Entrapment!?" (Fangame)?