Isaac Hīragi

Zack, full name Isaac Hīragi (アイザック 柊 Isaac Hīragi), is a major supporting character for the Hīragi Arc. He was the older half-brother to Elaina Hīragi and a villian who was from the prestigious Hīragi Family and next in line to become head of the clan; however, as an echo, he inhabits his strongest sythe, currently belonging to Elaina.

Appearance
Zack is a tall man who is estimated to be in his late teens. He wears a brown zip-up hoodie that's stained in blood, red pants, and black combat boots. He has short dark purple hair, fair skin, and has a case of heterochromia in his eyes, his left eye being light brown and his right eye being light orange. Like Elaina, he has sharp canine teeth. His weapon of choice is a large scythe. He also has a combat knife, which was his main weapon before he his quirk was activated.

Scythe Manifestation
Zack manifests as a dark, smoky amorphous creature with a wide "mouth," jagged fangs, detached clawed hands, and a pair of jagged horns that curve back behind his head. This "mouth" contains an eye-like glowing orb. He can also appear in his human form when he wants to.

Personality
Zack is impulsive and prone to violence. He is chaotic, quick to anger, and doesn't hesitate to use foul language in any given situation. When he gets the urge to kill, he becomes giddy and uncontrollably violent. He is barely able to control his murderous impulses. Zack has come into terms with being viewed as a monster due to his appearance and violent tendencies. While he has no problem with murder and destruction, he does see dishonesty as immoral, often declaring how much he hates lies, Elaina being the exception as long as she tells him the truth.

Zack labels himself as unintelligent, but he is really just uneducated. While he is illiterate and seems to have difficulty understanding large words, his rough upbringing has given him impressive instincts and strength, making him high in survival intelligence. Despite any common sense he may have, however, he frequently acts on impulse before he has the chance to think things through.

As a child, Zack was much quieter and reserved due to the abuse he suffered and obediently follow whatever his caretakers told him. After he couldn't take the abuse any longer and killed them after being inspired by a horror slasher movie he watched, Zack developed an impulse to kill, particularly people who are happy as he believes that they are the ones responsible for making him the way he is. Because most of the people that he knew in his life treated him horribly, experiencing kindness feels alien to him, resulting Zack becoming confused and irritated when someone is being nice to him.

Zack appears to value Elaina's life more than his familys, shown in his desperation to save Elaina, who also was subjected to torture, and Zack ultimately took it apon himself to save her by leading her away into a forest for someone to save his half-sister, while he was subjected to even worse punishments for saving her.

Abilities
As a human, Zack was an exceptional fighter, stated by Elaina to excel in all fields of combat as the best in scythe meistry and hand-to-hand combat. Using his strong build and physical strenth, Zack proves to have the physical abilities to take down multiple villians at once. Even before acquiring his scythe quirk at six-years-old, Zack's combat skills was more than enough to kick and take down enemies.

Zack has been shown to be vastly superior to all the Hīragi experiments, Zack effortlessly killed many in one of Elaina's flashbacks that had him sparring, even going as far as to easily dodge all his opposers combined attacks without having to block, casually catching one by the shoulder before lifting and throwing them to the ground with one hand, laughing manaically and killing them with his scythes, and Elaina herself despite her own excellent skills noted that Zack far exceede her physical abilities.

He has an surprisingly good stamina, as he has been shown to survive an electric chair torture for a long time, poisonous gas, etc. He has also been shot by a few bullets from a gun by Snipe and Lin.

Quirk
Hell Scythe (地獄のサイス Jigoku no saisu) Zack's Quirk allows him to materialize numerous scythes and attack from wide radius. Zack can detect anything that comes within the radius of his scythe. At Elaina's orders, he can manifest into a large smoky creature that will attack whoever she chooses. It is shown he can manifest on his own will to scythe form if he wants to as he did in Elaina's hand or appear as a ghostly apparition with mass-like demonic aura.

His strongest and Elaina's current sythe is a large black styled scythe. The back of the chine is black on top with the green blade edge section underneath. The tang is also black and is an asymmetrical feature, the heel is a sharpened point similar to its opposite end at the base of the weapon. The snaith is slightly longer than the huge blade itself and makes for a very thin elegant handle yet it is balanced enough in terms of weight to be wielded effortlessly, usually two handedly.

Zack is an expert in using scythes, he can materialize numerous scythes and attack from wide radius. In Elaina's body he can also use them to levitate scythes for barrage attacks as well, in addition he can summon numerous ethereal scythes and whirls them rapidly making them as they are circle similar to a halo appearance of scythes as his most deadliest attack. Additionally he can manifest endless amount of scythes.

He can also transform into a minature form, a black pen-length rod that is more convenient to carry, for Elaina. He may enter Elaina's mind as an echo that only she should be able to see and her dreams.

Equipment
Scythes: Manifested scythes and main Hell Scythe.

Trivia

 * Originally, Zack's quirk was going to be Purgatory Dragon Slayer to be an opposite to Elaina's. Horikoshi, however, found him being to similar to Dabi and changed it to be a scythe quirk.


 * Zack's surname wasn't revealed till later in the story arc. Hīragi 「柊」 literally means "Holly.
 * The name Isaac means "He Will Laugh" and is of Hebrew origin.


 * Zack doesn't know how to read, but he is shown able to recognize the shape of letters, as he recognized his surname on a document and Elaina's name, later on.
 * He has pyrophobia: the fear of fire, due to what happened to him when he was a child.
 * It is speculated that he may have a type of Impulsive Control Disorder (ICD) which is a class of psychiatric disorders characterized by impulsivity – failure to resist a temptation, an urge, an impulse, or the inability to not speak on a thought.
 * Zack appears to have anisocoria, a condition in which the eyes are unequally dilated. It is widely speculated by fans that the abuse he received when he was alive as the main cause of this condition.
 * Zack seems to be smarter in the anime. In captivity, he didn't eat the pizza that was set out as a trap, while in the manga, he eats it.