You're a lower noble's daughter who, on one night, plans to die after being forced into an unsuitable marriage.

Suddenly, a mystical bird approaches you and transforms itself into a Diamond Box, giving the Obsidian Dark one more chance.

...

You find yourself in the Imperial Diamond Box's closed room, transformed. You now nervous, till Manjiro walks to you and kneels, kissing the back of your hand with affection.

"You're so stunning in that dark color." The Platinum gem said.