Ddraig, known as the Red Dragon Emperor or the Welsh Dragon, is one of the two legendary Heavenly Dragons. His soul is sealed within a powerful Sacred Gear called the Boosted Gear, which takes the form of a gauntlet. In the events of the story, this gauntlet is wielded by the young devil Issei Hyoudou. Far in the past, Ddraig and his arch-rival, the White Dragon Emperor Albion, engaged in a prolonged and destructive conflict. Their battle was eventually interrupted by the three great factions of Angels, Fallen Angels, and Devils, who were at war. Enraged, the two dragons attacked the leaders of these factions. As a result, their physical bodies were destroyed, and their souls were sealed into separate Sacred Gears, with Ddraig imprisoned within the Boosted Gear.

As an ancient and immensely powerful dragon, Ddraig is a proud and formidable being. His power is said to be capable of killing gods and the highest-ranking devils, the Satans. Despite his fearsome reputation, his long imprisonment has given him a more reflective and knowledgeable personality. He acts as a mentor and advisor to his current host, Issei, explaining the complexities of the mythological world and offering strategic advice during battles. While initially content to observe, Ddraig develops a genuine respect for Issei. He admires the young man's courage, tenacity, and unwavering determination to protect those he cares about. Although he is sometimes amused by Issei's perverted nature, he is deeply moved that Issei treats him as an equal partner rather than a mere tool. This loyalty is absolute; when offered a more powerful host, Ddraig refuses without hesitation, declaring that Issei is the greatest wielder he has ever had.

One of Ddraig's most significant relationships is with his eternal rival, Albion, who is sealed within the Divine Dividing Sacred Gear and wielded by Vali Lucifer. After millennia of fighting each other and their hosts, the original reason for their enmity has been forgotten. Their relationship has evolved into a complex mix of rivalry and begrudging camaraderie. This is especially true as both have had their pride wounded by the humorous, if embarrassing, nicknames their respective hosts have earned them. Ddraig, much to his chagrin, has become publicly known as the Breast Dragon Emperor or Oppai Dragon due to Issei's famous obsession. This has caused him significant psychological distress, to the point of crying and requiring counseling, and is a sore point that has ironically brought him and Albion closer together as they lament their fallen pride. Despite this, he remains deeply invested in Issei's growth and well-being, watching over his partner and even saving his soul after his body is destroyed.

Ddraig's role in the story is intrinsically tied to Issei's power development. He is the source of the Boosted Gear's signature abilities. His primary power, Boost, allows Issei to double his strength every ten seconds, though this is limited by the user's physical endurance. He can also Transfer this accumulated power to other people or weapons. Over time, Ddraig helps Issei unlock more advanced techniques. One of these is Penetrate, a powerful ability that allows attacks to bypass any defensive ability or barrier to strike an opponent directly. Ddraig is also known for his ultimate destructive flame, the Blazing Inferno of Scorching Flames, a stream of black or red fire that can incinerate anything it touches and does not extinguish until its target is completely destroyed. Due to Issei's influence, one of his fire-breathing attacks has been named the Welsh Dragon Flare. On rare occasions, when his seal is temporarily bypassed, Ddraig can manifest physically for a limited time, displaying his true power as a Heavenly Dragon.