Tannin

Known as the Blaze Meteor Dragon (魔龍聖（ブレイズ・ミーティア・ドラゴン） Bureizu Mītia Doragon?) and the Devil Dragon Saint (魔龍聖 Maryūsei?), he is the Purple Dragon and a former Dragon King that reincarnated and became an Ultimate-Class Devil. He trained Issei in the mountains in order to for him to attain his Balance Breaker. Tannin has a close relationship with Issei ever since he trained him, allowing him to call him "Old Man Tannin" (タンニーンのおっさん Tan'nīn-ossan?). He became a Devil to gain access to the Underworld in order to train young Dragons and preventing their race from going extinct, as the majority of them are either dead, missing, or sealed in Sacred Gears. It is later revealed in Volume 14 that he is Mephisto Pheles' Queen, making him the first male Queen piece.