Ze-Kalios the Moon Dragon (月竜 « ゼ＝カリオス »), also known as the Dragon of Harmony, is one of the Five Dragons of Altago. His role is to guide all souls to their next life. He is revered by the Tribe of Iska, one of the Five Tribes of Altago.
Ze-Kalios resides in the Moon Sanctum whose altar is actually located in the Hidden Village of Iska that was created in a pocket between the world of living and the dead. The incantation needed to access his sanctum has been long lost but Adol was able to use Tia's Hairpin to gain entry inside.
Unlike the other dragons, Ze-Kalios has two forms in battle where he enrages after a certain amount of damage is dealt to him. Upon defeat, he finally revealed the true role of the Dragon Warrior to Adol. Ze-Kalios would also grant his remaining power to Adol by condensing his essence into the Moon Dragon Stone.
After the defeat of the Root of All Existence, Ze-Kalios explained that since the five dragons are but another side to the root, so too will they disappear. As the five dragons had sustained Altago previously, they endowed that task to the people of Altago from now on.