He first appeared in volume 2 of the novel.

Spoiler

Also known as the Welsh Dragon or the Sekiryutei (Red Dragon Emperor). One of the Two Heavenly Dragons along with the Vanishing Dragon Albion. The two dragons, with powers rivaling to gods, started fighting a long time ago for reasons even Ddraig forgot. At that time, the angels, devils and fallen angels were waging a war against each other. The 3 factions believed that the fight the two dragons were having could spell disaster to the whole world so they set aside their war to stop the fight and seal the two dragons. He now resides inside Issei's Sacred Gear as a result of being sealed in it.