Arc-En-Ciel

Arc-En-Ciel is one of the Next Dolls created by Promestein. She was specifically designed to defeat Granberia, easily overpowering the cursed swordswoman. Interestingly, similar to Granberia, Arc-En-Ciel loves fighting and a good challenge.

At the end of Chapter 2, after Ilias declares war and Promestein lays siege on the Category:Monster Lord’s Castle, Arc-En-Ciel easily overpowers Granberia and mocks her, though Luka tries to deny this by saying that the chimera was only able to defeat the Heavenly Knight because Luka weakened her in a fight moments ago.

Continuing on in Chapter 3, after Luka defeats Cupid, Valkyrie, and Archangel Ariel, Alice tries attacking Promestein, the leader of the attack. Arc-En-Ciel steps in, but Promestein stops her as there are already countermeasures against the Monster Lord, much to the chimera’s dismay. After Promestein seals Alice as well as Luka’s powers and then move on to capture him, Tamamo suddenly shows up with Giganto Weapon who mows down the chimera forces, but is made of short work by Arc-En-Ciel, who is a bit disappointed by its frailty; Promestein reassures her that it used to be stronger a long time ago. Shortly afterwards, Tamamo scares the Chimera Beasts with her aura alone, prompting Promestein to call a retreat, much to Arc-En-Ciel’s dismay.

Trivia

 * Her name is French for rainbow, though how this relates to her character or design is yet to be seen.
 * Another possible explanation for the name is a degeneration of the word Archangel which is, when said, pronounced as Arc-En-Ciel.
 * Also, given that an archangel as a "high-ranked" angel and should be the counterpart of Granberia, the strongest of the Four Heavenly Knights, it is likely that this monster is some sort of boss monster (not confirmed, so not categorized as one).