Tannin

Tanninim is the Purple Dragon and former Dragon King, known as the Blaze Meteor Dragon, he is reincarnated into an Ultimate-class Devil. He is the Dragon that trained Issei to obtain his Balance Breaker.

History
He became a Devil so that he can gain access to the Underworld in order to train young Dragons and preventing their race from going extinct as majority of them are either dead, missing or sealed in Sacred Gears.

Plot
He trains Issei in one of the mountains in the Underworld to help him achieve Balance Breaker.

Powers & Abilities
A former Dragon King and Ultimate-class Devils, Taninim is an extremely powerful Dragon that is on par with the Four Satans. He is also ranked top 10 in the Rating Games.