Amber

Amber is a construct, a copy of the Dwarven princess of Idonokan, which is now nothing but ruins. During the last abyss war, the Dwarves were thought all killed, with their country being one of the hardest hit. Any survivors disappeared underground and withdrew from the world.

At some point, Amber was created and given the Ring of Earth. She was somehow (assuming the same way she can later open a portal between worlds) transported to Earth and enshrined for hundreds of years as a divine being in Satou's home town.

When the Ring King's party failed to defeat the horde attacking abyss monsters and Abyss Knight while searching for the last ring, the Great Sage opened a portal and sent Satou and the princesses to Earth to keep them safe.

While back home, Satou found Amber in the old run down shrine. She offered her ring and her entire self to him, as that is what she was made for. He hesitated to take the final ring, worrying for the girl's and his own safety. She said she is fine with whatever he decides, as she has already been waiting hundreds of years. After getting over his fears, he accepted her powers and she was able to open a portal back to the other world. She becomes his 5th wife.

She states there is no need to deepen the bond between her and the Ring King. She was created to be his and their bond started at 100%. She does not appear to have a drive like some of the others to have Satou pay attention to her. She also mentions she has the ability to reproduce, containing the last DNA of the Dwarves within her.