Cusack

is a high-ranking demon of the Demon Clan as well as Zeldris' former master.

Appearance
Cusack is a tall and muscular man with black hair that has bangs extending to cover his right eye and end in a long ponytail braided. He also sports a very long mustache that reaches his pectoral section and a small goatee.

His demon mark consists of two black arrows extending below his left eye and a spot of darkness on his right shoulder that extends further around his arm.

His attire consists of a skin tight white vest shirt with no sleeves and black pants in knee high boots. On his forearms are detached sleeves.

Personality
Cusack is very confident in his abilities, requesting Zeldris' permission to exterminate the Seven Deadly Sins, former Fairy King Gloxinia, and former Giant King Drole himself.

Like Chandler's attachment to Meliodas, Cusack is very much attached to his student, Zeldris, expressing the utmost confidence and loyalty to the Demon King's third son. This extends to the point that he believes Zeldris to be worthy of reigning the Demon Clan next instead of Meliodas or even Estarossa. This attachment led him to express complete anguish and shame along with rage when he ranted about how he failed to protect Zeldris when Meliodas pinned his two brothers down upon his arrival to Camelot.

Despite his loyalty to the Demon Clan, Cusack displays some amount of criticism to the Demon King where he goes as far as to call the Demon King being too compassionate with Meliodas in spite of his betrayal. He also has some wariness towards Estarossa due to never being able to properly read him though acknowledges the Demon King's second son's power to be probably greater than his student's.

In battles he swiftly brings down opponents as efficiently as possible as shown with Cath but is also rather patronizing towards them if he notices distinct flaws in their actions as he lectured Arthur Pendragon on why he failed to win the battle like intended.

Cusack is shown to be proud of his mustache, claiming it as the number one mustache in the Demon Clan; he even was shocked after realizing half of it was cut off in a fight.

3,000 Years Ago
Long before the Holy War, Cusack trained Zeldris, the third son of the Demon King, in combat.

Eventually he along with the rest of the Demon Clan were sealed away into the Coffin of Eternal Darkness by the allied clans (Goddess, Giant, Fairy, and Humans).

Current arc
After Zeldris claims that he is the most worthy to be the next Demon King, Cusack, along with Chandler, walks into the scene. Cusack says that he can see Zeldris on the throne of the Demon King, and he wants to help him by offering his strength. He then bows down and explains why it took long for them to get out of the seal. He apologizes for eavesdropping Zeldris' conversation with the Demon King, and requests Zeldris' permission to exterminate the Seven Deadly Sins, Gloxinia, and Drole. Zeldris tells him to stand and says that Cusack and Chandler will fill the void of the other nine Commandments. He replies that he is not worthy of his praise. Chandler tells how jealous he is that Zeldris and Cusack get to have a student-master reunion. Cusack tells him not to worry because they get to serve the demon army again, and reveals that he is Zeldris' former master and Chandler is Meliodas'.

Abilities and Equipment
As Zeldris' former master, Cusack is an extremely powerful demon. Zeldris states that along with Chandler, he is more than enough to fill the nine vacant spots of the legendary Ten Commandments. Arthur Pendragon states that his power is comparable to that of his former student.

Like all members of the Demon Clan, he is able to manipulate the mysterious jet-black power of darkness for a variety of purposes, such as forming wings for flight and regeneration.He's able to swiftly regenerate from being bisected by Excalibur from the shoulder to the hip and his arm cut off easily.

Cusack possesses immense levels of endurance, shown when he remained mostly unfazed from having been pierced through the abdomen by Meliodas’ darkness in his Assault Mode, though he was immobilized till the former recalled it back. He also possesses immense physical strength, being able to completely shatter a stone wall with a single grip.He later easily broke free from Merlin's Icicle Castle just by moving.

Abilities

 * This ability allows Cusack to link with someone through eyesight. When activated, his eye and the eye of his target glow the same color and the person does the same movements as Cusack.

Weapons

 * Twin Hilts: Cusack carries two identical hilts on both sides of his hips.

Zeldris
Cusack strives to impress Zeldris in any sort of way and admires him calling him the most worthy of being the Demon King. He is also envious when Zeldris praises someone else as he said that if his love for Zeldris was on the line then he would not lose to anyone. While not conducting any missions, Cusack is shown attend to Zeldris like a butler and is shown to be comically wounded when he denounces his tea and advice as terrible. He also displays complete rage when Meliodas brought Zeldris down with his darkness and was ready to attack Meliodas for it.

Estarossa
Cusack does not get along with Estarossa as he is quite weary of his actions. He does not understand him or his feeling and action just like Meliodas, but deduces that there is something out of place from him. However he does acknowledges his strength as he believes his power exceeds even Zeldris.

Chandler
Cusack seems to get along well with Chandler as they talk and walk casually together. We do not know their history but it seems they have a mutual relationship. However Cusack does seem to get annoyed at Chandler when ever Meliodas is mentioned as it upsets the latter as becomes a cry baby and obsesses over Meliodas.

Meliodas
Cusack does not seem to be forgiving Meliodas for his betrayal the Demon Clan, but it's unknown whether he cares about him or not when he said to Zeldris that the Seven Deadly Sins should just let Meliodas go and let themselves be killed.