Satou Haruto is the new Ring King when he was chosen by Krystal to be her husband, and in turn being given the Ring of Light. He's known Krystal since they appeared 10 years ago alongside Alabaster.

When Krystal returned to her world due to having to marry Marmaru-Gias Gisaras to unite the Nokanatika Kingdom and the Empire, Satou entered the teleportation gate and appeared in the other world. When he appeared in the middle of the wedding ceremony between Krystal and Marmaru, a creature from the Abyss appeared. During the confusion, Krystal kissed him and bestowed the Ring of Light to Satou, making him the Ring King.

He then travels with a number of people on and off, but mostly Hime and Gramps. As he marries the other Ring Princesses, they join him in his travels and gains their rings as well.

(Source: Tales of Wedding Rings Wiki)