Devack

Demak 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 but it is unknown why. 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, the Demonic Monkey King Zeman.

Dark Signers
When the powers of the Dark Signers, Misty Lola and Carly Nagisa manifest simultaneous their Earthbound Immortals' lines are etched out around the Arcadia Movement building. A Security wagon transporting Greiger is within the lines. After "Earthbound Immortal - Ccarayhua" is Summoned, Demak appears and frees Greiger from their custody, breaking Greiger's handcuffs with "Axe of Despair".

He appears to have some connection to him despite Greiger not knowing Demak. Still, Demak invites Greiger to join the Dark Signers if he wants to get his revenge on Yliaster for destroying his village, which Greiger apparently accepts. Demak made a stop to pick up the newly awakened Carly and Misty after their marks disappeared from the city and sky, then proceeded to head back to Satellite.

He finally shows himself to his rivals, the Signers, after Rudger'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 Five 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 4 safety switches of the Old Momentum. As Luna and Leo are taken by Trudge to where Demak is, she has instead gone into the Duel Monster Spirits World in order to free Ancient Fairy Dragon from her imprisonment with the the help of her servant, Regulus. However, when Leo finds the safety switch, Demak 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 Demak'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. Demak is quickly able to inflict damage to his opponent at the start, but Leo counterattacks with Power Tool Dragon, his ace card. Demak then summons the Dark Synchro Monster, Monkey King Zeman. Eventually, Leo is able to destroy his Dark Synchro Monster, utilizing his Power Tool Dragon. However Demak 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 Disc and the twins duel together.

However, Demak summons Ancient Fairy Dragon, and later sacrifices her to prevent Cusillu's destruction. This ultimately backfires for him, as Luna not only uses Leo´s Respect Synchron to summon Ancient Fairy back, but also activates her Dragons´ special ability to destroy Closer Forest thus negating all of Cusillu´s effects rendering it useless. Then Luna assaults it with Power Tool Dragon in order to weaken the Monkey God, in order to destroy it with Ancient Fairy Dragon's attack. Thus at the end Demak loses. He then collapses into a pile of dust.

Deck
Demak plays an Ape-themed Beast Deck, in which his Ape monsters can gain an additional Type. For a short period of time, he had under his control Luna's Ancient Fairy Dragon card, as well as total control over the seal holding her Dragon's Spirit.