Ddraig

ドライグ

Ddraig is one of the Two Heavenly Dragons, the Red Dragon known as the Welsh Dragon and Red Dragon Emperor who resides within the Boosted Gear, wielded by Issei Hyoudou. He is feared among the Angels, Fallen Angels, and Devils for his destructive powers, which are said to be able to kill both Gods and Satans.
In the past, Ddraig fought against his arch-rival, the Vanishing Dragon, Albion, which was interrupted by the Angels, Devils, and Fallen Angels who were at war at the time. In anger, both Dragons attacked the leaders of the Three Factions, which led to their bodies being destroyed and their souls being sealed into two separate Sacred Gears, with Ddraig being placed inside the Sacred Gear "Boosted Gear".
At one point before he was sealed into the Boosted Gear, Ddraig rampaged in the Realm of the Dead along with Albion. He also fought against Grendel sometime in the past.
Ddraig appears in the beginning of Volume 2, introducing himself to Issei. He reappears after Issei's defeat at the hand of Riser Phenex, telling him about the "White Guy".
In Volume 3, Ddraig has a conversation with Issei as the two eventually start a teacher-student relationship, with Ddraig giving advice to Issei during fights such as the fight against Kokabiel and the crisis with the Khaos Brigade in Volume 4. Ddraig also teaches Issei about many things regarding the mythological world.

Spoiler

Being sealed within Boosted Gear, Ddraig has yet to show his true personal power and full array of abilities. It is known that he can multiply and transfer power, and has the common powers of Dragons, but it is unknown if this is the full extent of his abilities.
When his powers are unsealed, Ddraig is stated to have power enough to rival or kill a God or a Satan.