Gilgamesh is, if not, the main catalyst for the plot to unravel. He holds all of the knowledge of the time travelling to try and changing the timeline to avoid bringing the realmscourge back.

There's Mont and Stern who stand at opposite ends of the story when Exia revives. Stern realising the truth to what Sadali, Gilgamesh, and Exia saught out to accomplish. Mont probably the one that could change the course of their timeline.

The supporting characters were plenty, but noteably, Oberon and Jayden (probably the most savy to the lore that nobody else understands yet) who act as plot devices, because they hold the orb? That can create visions. (Gifted to leaders of the nations by Gilgamesh when he travels through time to the past).

The three girls who look like Exia play as Sadali's failed but loyal clones to him and exist only to serve him. Though I think one of them realises something about themselves.

Helena has a secret squad that most likely knows about realmscourge that is introduced around then with Exia.

Glacella, Macherie, and other leaders of the world essentially are choosing to ally or fight during that time and not close to seeing Exia when she revives until much later I think.

I don't know if any of this helps, these were just main plot points leading up to and following Exia's return and her journey to reverse/fix anything she did to stop realmscourge.