Duel is a mysterious demon. Though he initially went berserk in the past, he later seemingly became devoted to protecting Mari, but his true motives in the present remain vastly unknown.
Sometime before the founding of Kounat, the League of Demons initiated a surprise attack on the ancient demons, leading to the Ancient War where the ancient demon's hero, Duel went berserk upon the death of his lover, Edna. He annihilated everyone in disregard of whether they were allies or enemies, and because of that the war was practically terminated.
Duel also saved Mari from the explosion that annihilated Kounat for his own purposes, which are still unknown.
Zero's story description states that Duel created one of the three legendary demon weapons, Eclipse, and used it in the Demon Wars. However, for some reason, the lust of destruction grew within, killings the demons that stand in his path. A wizard, Oz, created Grandark to counter Eclipse.
In Thunder Hammer, he makes his appearance to steal the Chase's Deity Orbs. Zero then appears and attempts to attack Duel, who recognizes him as "one of them" and leaves. Grandark stops his master, warning him that Duel has become too powerful to defeat thanks to his Transcendence and the new demonic tribe he had allied with.
Duel appears in Kounat's Collapse after Ashtaroth's defeat and prevents Ashtaroth from taking control of Mari, who had been teleported there by the Aernas Hammer. He then takes custody of the Klara Libri, and removes Mari's memory of her past before leaving.
Duel attacks by firing piercing blue orbs and a homing black orb. Note that the homing orb cannot be destroyed by simply hitting it, and must be outrunned.
He is capable of teleportation, yet he must open a black hole to do so.
It should be noted that Duel cannot be defeated (all damage does 1 on him), and will only leave once players have hit him enough.
- Duel is also known as Duel Pone Zec Avenger.
- Despite the fact that Duel cannot be defeated, he drops items and even a card, oddly a trait not shared by Ashtaroth.
- On Duel's back are large swords and what appears to be a pike. They are called Eclipse and Transcendence.
- In Thunder Hammer Champion Mode, there is a chance that Duel will drop the swords, which are treated as accessories. Also, the Eclipse and Transcendence can be made out of both swords, 11796 Gems, and 750,000 GP with a level 75 character. Note that the crafted material is only limited to the selected character.
- In Thunder Hammer, Duel appears to be holding back, as his only intentions are to steal the Deity Orbs from the Grand Chase rather than killing them. This is also supported on how he is a demon like Dio (usually possessing unimaginable strength) and he does not use his swords on his back.
- Strangely, despite the fact that Duel has been shown to be antagonist (by aiding Ashtaroth and according to Zero's description), the demon suddenly assists the Grand Chase upon his arrival in Kounat's Collapse by denying Ashtaroth the Aernas Hammer and then proceeds to ease Mari's suffering of the Klara Libri by erasing her memories of it.
- Strangely, despite supposedly being devoted to protecting Mari, he attacks her and her comrades in Thunder Hammer.
- According to the Naver Blog, Duel and Dio are antagonistic towards each other for reasons yet to be revealed. In Grand Chase Dimensional Chaser, it is revealed that Duel killed Dio's father.
- According to the Naver Blog, Duel is an "other" member of the Twelve Disciples whose duty as the Gate Keeper is to protect the Key Master, Mari.
- According to a Brazilian article, Duel tried to activate a time machine in hopes to prevent Edna from dying and needed something from Mari to do so.
- There is a chibi variant Pet of Duel. This Pet, known as Duelly, attacks by transforming into his original size and using a black orb similar to Thanny Boy's Shadow Explosion.