Devack, known as Demak in the Japanese version, is one of the Dark Signers. He wears the black robes of the Dark Signers accented with gold stripes. He has great hatred towards the Signers. His dark mark is the monkey. He frees Greiger from his prison transport, offering him the chance to fulfill his desire for vengeance. He is also responsible for the Minus Curse that infects the Spirit World with the help of his subordinate, Zeman The Ape King.
It is unknown why Devack joined the Dark Signers, though he appears to have been one of the earlier members. He first appears out of the original Ener-D Reactor arriving by helicopter to free Greiger from custody, as his transport guard's souls have been absorbed by "Earthbound Immortal Cusillu". Roman then invites him to join them and get revenge on Rex Goodwin for the destruction of his home town. They later pick up Misty Tredwell and Carly Carmine to return to the Ener-D reactor.
He finally shows himself to his rivals, the Signers, after Roman's Shadow Duel against Yusei concludes. The battle between them and their rivals as their final battle is about to commence.
Before their departure he shows Luna that he possesses her "Ancient Fairy Dragon" card, and so in order to get it back she must fight him. Later on, as each one of the Signers is set on defeating their respective opponent, they split up to each of the four control units of the Original Ener-D reactor. As Luna and Leo are taken by Trudge to where Devack is, she has instead gone into the Duel Monsters Spirit World in order to free Ancient Fairy Dragon from her imprisonment with the the help of her servant, Regulus. However, when Leo finds the control unit, Devack approaches him while the Monkey geoglyph materializes. Leo then bravely challenges him, stating that if he defeats the Dark Signer then Luna won't have to Duel.
Despite Devack's warning that Leo won't be able to save himself and his sister, as he is not a Signer, the duel is still set on. Devack is quickly able to inflict damage to his opponent at the start, but Leo counterattacks with "Power Tool Dragon", his ace card. Devack then Summons the Dark Synchro Monster, "Zeman the Ape King". Eventually, Leo is able to destroy his Dark Synchro Monster, at the exact same time Regulus destroys Zeman in the spirit world, by utilizing his "Power Tool Dragon". However Devack summons his true ace card, "Earthbound Immortal Cusillu". He attacks directly, but Leo prevents himself from being defeated. Then, Luna arrives, takes Leo's Duel Disk and the twins duel together.
However, Devack Summons "Ancient Fairy Dragon", and later sacrifices her to prevent the destruction of "Cusillu". This ultimately backfires for him, as Luna uses Leo's "Respect Synchron" to Summon "Ancient Fairy" to her side and uses its effect to destroy "Closed Forest" thus negating all the effects of "Cusillu" and rendering it useless. Then Luna assaults it with "Power Tool Dragon", weakening it and destroys it with "Ancient Fairy Dragon". At the end, Devack loses, making him the first Dark Signer to fall.
Regretting his defeat, Devack turns into dust while asking Roman for forgiveness for failing the Dark Signers. (In the English version, he tried to drag Leo and Luna with him to the Netherworld, but failed when Yusei came in and saved them).
When Rex is defeated in the final battle, and the King of the Netherworld is destroyed, Devack is restored to life, along with the other Dark Signers except for Roman and Rex himself. It is currently unknown whether he will have memories of his time as a Dark Signer. At this point, it's presumed he returned to his old life, before dying.
Despite the others, Devack is the only Dark Signer whose character and transformation into a Dark Signer was never explored. He is also the only one who was not shown repenting for his crimes as a Dark Signer. Furthermore, according to the Yu-Gi-Oh! 10th Anniversary Animation Book, Devack was originally intended to be from a demonic worshipping cult and was sacrificed for their God's, but was reborn as a Dark Signer. It is unknown why they decided not to explore this in the anime.