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. His dark mark is the monkey. He frees Greiger from his prison transport, offering him the chance to fulfill his desire for vengeance.

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 to Luna that he possesses her Five Dragon card, and in 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. However as Luna and Leo are taken by Trudge to where Demak, she has instead gone into the Minus World in order to free Ancient Fairy Dragon from her imprisonment. With the the help of her servant, Regulus. However as Leo finds the safety switch, Demak aproaches 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 as he's not a Signer. Still the duel is set on as Demak is quickly able to inflict damage to his opponent.

Deck
Two other Monsters remain to be confirmed to be Beast-Type Monsters that resemble apes, as they were on his opening hand. Leading up to the conclusion that his deck may be an ape-themed Beast Deck. For the meanwhile he also possesses Luna's Ancient Fairy Dragon card, as well as total control over the seal holding her Dragon Spirit.