Isaac Hīragi/Synopsis

History
Zack was concieved through artificial insemination using the seed of Gray Hīragi in highly gifted women, when he was born his mother took him away to live with her. He was born in a poor and abusive family as he never received any sort of education, resulting him unable to read nor write. When he was still a child, his mother's lover set him on fire in an attempt to kill him, but he survived. Not long afterwards, his mother wrapped him with cloth and left him to die. That was until one day, when someone left the TV on, showing a violent horror film. Zack took inspiration from it, seeing it as a way to solve his problems, and murdered his mother and her lover by repeatedly stabbing them before mutilating their bodies, later burying them in the backyard. He was later taken in the care of a the Hīragi Family before the authorities arrived.

Showing great promise from birth, Zack followed the traditional Hīragi training, which was imparted to him by his older half siblings. He had already mastered many killing techniques at a tender age and was set to be one of the best assassins the family had ever produced. At the age of three, he started training in tailing that routinely put his life at risk. Eventually gaining his own floor in the Hīragi Sacrifice Building.

Soon after gaining his own floor, his half-sister, Elaina Hīragi, was conceived through the same experimentation. At some point (or even earlier), Zack began having nightmares where the demon weapon spoke to him from the shadows, and its voice would get louder as he continued to grow. When he learned that Elaina was also having the same dreams as him, Zack tells her to not tell anyone or they would be used as experiments again. While resting after having 'fun' on his floor, Zack witnessed Elaina's offensive quirk being activated and the death of Aoi, later taking the blame for her death.

At some point, Elaina and Zack were introduced to All for One under the alias of Satoshi. All for One saw Elaina's great potential and Zack really wanted to kill him, losing himself to his impulses and attacking with fail. Elaina as a defense requested Zack's Hell Scythe and protected him All for One's defensive strikes, although he only seemed to be playing, which further frustrated Zack. Elaina, also getting lost to her impulses attacked All for One and managed to evade all his defensive motions and injure him with the scythe. Causing him to congratulate her and mention how dangerous she could be.

When Zack was 7 he went out to find a gift for Elaina as she had healed him of his burned flesh. While out, he killed a drunkard. He found an antique earing and was sharpening his knife to go kill the employees before stealing it, even though the Hīragi Family would have payed for it if asked. As he was heading over to the shop he was encountered by All for One, who asked what he was doing outside the estate. After hearing his plan All for One offered to go get the earing for him. Elaina was happy to see her brother return and said that the earing is precious to her as it was a gift from her beloved brother and Satoshi. A flier with the reported destruction of the shop was seen at the end.

When Elaina was 10, Zack suggested they run away as Elaina's quirk has gotten stronger and more dangerous for her to be around. He had Elaina make a plan as he says that he is to dumb and they proceed to argue about who has more knowledge about the outside world. They escape the next night, Zack carrying her most of the way and then places her down to her confusion. Zack tells Elaina that if he goes it'll be more dangerous as he is a potential heir and she is worthless. Elaina begins to cry sying that he promised they would stay together. Zack comforts her and apologizes for lying. Elaina believing she understands asks about him and he replied sayiing he'll be fine. Elaina clutched onto him and repeated how she wanted to stay with him, leading to him physically stricking her away and then running back.

Years later, his recent killings became quite famous, making him a currently wanted serial killer and criminal. His identity is still unknown so the tabloids call him the Reaper.