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 vengence.

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 a Equip Spell card, "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. Greiger apparently accepts. Demak made a stop picking up the newly awakened Carly and Misty after their marks disappear from the city and sky and head back to Satellite.

Although Demak hasn't dueled yet, it has been confirmed he is to duel against Leo in episode 48, as this non Signer takes him on after having to replace Luna whom has instead traveled into the Spirit World in search of Regulus.

Deck
The cards in his deck are monkeys and his Monkey Earthbound Immortal, he has also been seen using Axe of Despair to free Greiger's handcuffs in the episode 39.