Ddraig
Race: Dragon (Heavenly Dragon)
First Appearance: Volume 2
Ddraig is one of the Two Heavenly Dragons—specifically the Red Dragon, also known as the Welsh Dragon and the Red Dragon Emperor. He's currently sealed inside the Boosted Gear, which Issei Hyoudou wields. Among Angels, Fallen Angels, and Devils, he's got a serious reputation for destructive power that's supposedly capable of killing both Gods and Satans.
His whole rivalry with Albion, the Vanishing Dragon, goes way back. The two were locked in constant conflict until the Three Factions' war interrupted them. Both dragons got angry enough to attack the faction leaders directly, which resulted in their physical bodies getting destroyed and their souls trapped in Sacred Gears—Ddraig ended up sealed in the Boosted Gear as a result. Before that happened, he and Albion actually tore through the Realm of the Dead together, and at some point in the distant past, he also fought Grendel.
Ddraig first introduces himself to Issei at the start of Volume 2, popping up again after Issei's loss to Riser Phenex to fill him in on "the White Guy." From Volume 3 onward, the two develop a mentor-student dynamic, with Ddraig offering guidance during major fights—like against Kokabiel and later the Khaos Brigade crisis. He becomes a key source of knowledge about the mythological world too.
Since he's locked away in the Sacred Gear, his true power remains mostly unknown. He's shown he can multiply and transfer power, plus he has the standard dragon abilities, but whether that's all he's got is still a mystery. When his powers do get unsealed though, he's supposed to be strong enough to go toe-to-toe with or even kill a God or Satan.
Content compiled by AnimeList.moe from publicly available sources.