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 red 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, black pants and high knee boots. On his forearms are detached sleeves.

Upon revealing his true form, Cusack grows much larger in stature and his arms grow longer and hands become larger, enough to hold both his twin hilts in between his fingers. His hair and mustache also grow and turn white. The color of his eyes are inverted. He also grows large wings from his shoulders. His demon mark is larger that extends down his cheek with the arrows converging together and a mark grows on his chest forming a cross.

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 all by 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 being a loyal vassal of the Demon Clan, Cusack is shown to be quite critical towards some of them as he outright called the Demon King being too compassionate with Meliodas in spite of his betrayal. He also denounces Chandler as being imcompetent when he and Meliodas returned to Camelot with Elizabeth still alive despite it being Meliodas' orders. Also Cusack has some wariness towards Estarossa due to never being able to properly read him, though he does acknowledge 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 will toy with them should he be bored enough and also if he is curious about them. He is also rather patronizing towards them if he notices distinct flaws about them 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 and was even shocked after realizing half of it was cut off in a fight.

History
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).

Prelude to the New Holy War 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 so long for them to get out of the seal. He apologizes for eavesdropping on 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 says 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 once again, and reveals that he is Zeldris' former master and Chandler is Meliodas'.

When Zeldris reveals his father's plan to retrieve Meliodas to make him the new Demon King, Cusack is reluctant, wondering what the Demon King's intent may be after that, to which Zeldris responds in ignorance. When Estarossa awakens momentarily from his coma, Cusack shows concern for his presence, knowing the mental instability that is next to the great strength of his disciple's brother.

After the departure of Chandler to rescue Meliodas, Cusack stays in the castle to stay with Zeldris, something that the latter finds suffocating, although this does not bother Cusack. When Zeldris mentions that the Seven Deadly Sins should not be underestimated, Cusack claims that Chandler is still in a position to finish them. During a tea that Zeldris finds disgusting, he comments with Cusack that things will get out of control once Chandler returns to his true form.

Abilities and Equipment
As Zeldris' former master, Cusack is an extremely powerful demon. 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.

Zeldris states that along with Chandler, Cusack is more than enough to fill in the nine vacant spots of the legendary Ten Commandments. According to Arthur Pendragon, Cusack's power rivals that of Zeldris'. His aura was so powerful that it completely terrified the human souls inside Excalibur.

Cusack possesses immense levels of endurance, as 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 and 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.

Prelude to the New Holy War arc

 * Arthur Pendragon vs. Meliodas, Zeldris, Cusack & Chandler: Win

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.

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 of Meliodas' betrayal of 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.

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.