Dumon is the only Barian Emperor that never Dueled in his human form, or against Yuma Tsukumo.
He was by Jake Paque.
Long ago, Dumon was a knight who served to protect a kingdom he lived in along with his pegasus, Mach. He was friends with the ruler of the United Lands of the Poseidon Ocean, Nash. When Nash's kingdom was attacked by Vector's forces, Dumon arrived to assist. He was targeted by Vector's archers, but he and his pegasus were too quick to be hit by the arrows. Nash's sister Marin was taken by the ocean god that Vector Summoned, Abyss. Dumon attempted to save her, but Marin pitched herself from Abyss' hand, sacrificing herself to call forth another god. Dumon then stood by Nash's side as he mourned his sister's death. He then assisted Nash's army in assaulting Vector's once more. The assault led them to a town Vector ravaged on his way, which sickened Dumon as he allowed his people to live in such conditions. As the war between the two kings prolonged, Dumon and Nash planned to confront Vector and his military once they reached his location. Dumon fought and lead Nash's army in the battle and was the only known survivor. At one point in his life, Dumon obtained a card called "Number 44: Sky Pegasus". Eventually, he returned to his homeland, but learned about how his fellow knights had grown arrogant and plotted to usurp the King. Dumon and Mach intercepted the other knights and tried to reason with them, but Dumon failed and the knights turned their swords on him. Since Dumon was unwilling to kill his former comrades, both master and steed were eventually defeated and killed. However, Dumon's fellow knights regretted their actions and they buried him and Mach together. The knights had been hyponotized by Don Thousand in an effort to cause Dumon's rebirth as a Barian.
Among his allies, Dumon is shown to be the most level-headed among them and was at one time their leader. He is shown to care about his fellow Barians greatly, often voicing his high opinion of them and usually letting them know that he has faith in them before seeing them off on their mission. Dumon is very loyal to the Barian World and prioritizes its safety above all else. Likewise, he is extremely loyal to the original leader of the Barian World, Nash, going to great lengths to find and restore him to his original position within the Seven Barian Emperors. This appears to have stemmed from their previous human lives. Dumon is also shown to intimidate the other Barians, especially Alito, whom complained about the mission he received, but complied when Dumon used a strict tone. He also seems to dislike confrontation as he often puts an end to arguments and fights between his comrades, though this did not stop him from threatening to put an end to Vector with his own hands if the latter did not stop acting alone. He is not averse to praising his enemies, as he did during his Duel with Shark, where he complimented the young man's determination and Dueling skills.
He was also willing to sacrifice his life in order to protect his comrades, as seen when he was trying to used his "Celestial Rage" to end his last Duel against Vector in a draw, as well as using "Sacred Shield" to protect Marin, even though it meant his death.
Dumon's name is derived from Alpha Ursae Majoris, also known as Dubhe, a star located in the Big Dipper asterism. His alias in human form, "Nash" is derived from another star in the same asterism - Eta Ursae Majoris, also known as Benetnasch.
Dumon, like the other Barians, has been shown to teleport using portals. Dumon has also been shown to generate a large energy ball which he can use to fly. Dumon is one of the few Barians who can stay in his true form, even on Earth (though he was in close proximity to Number ruins at the time). The only exception is when his Baria Lapis is damaged.