Zack is a tall, lanky man, estimated to be in his twenties according to records discovered by Rachel Gardner on Floor B4. He wears a bloodstained brown zip-up hoodie, red pants, and black combat boots. His entire body is wrapped in bandages to conceal the extensive burn scars covering his skin. He has short, messy black hair, anisocoria (unequal pupil sizes), and complete heterochromia—his left eye is light brown, while his right eye is gold. His primary weapon is a large scythe, though he originally wielded a combat knife before acquiring it.

In his childhood, Zack’s hair was even messier, and he wore a dirty short-sleeved shirt with short pants. Before sustaining his burns, he had a fair complexion, which later became light brown beneath the bandages due to the severity of his injuries.

Zack is impulsive, aggressive, and prone to extreme violence. Chaotic by nature, he is quick to anger, frequently uses foul language, and becomes giddy and uncontrollably violent when overcome by the urge to kill. He openly accepts being perceived as a “monster” due to both his appearance and his violent behavior. While he has no moral qualms about murder or destruction, Zack holds a strong personal disdain for dishonesty, often proclaiming his hatred for liars.

Although he describes himself as unintelligent, Zack’s lack of formal education is the primary cause of his illiteracy and difficulty with complex vocabulary. His harsh upbringing has instead equipped him with sharp instincts, significant physical strength, and strong survival skills. However, he is prone to acting on impulse before thinking matters through, often leading to reckless decisions.

In his childhood, Zack was quiet, reserved, and obedient, following orders from his caretakers without question. Prolonged abuse, however, eventually pushed him to his breaking point. After watching a slasher film on television, he killed both caretakers, marking the start of his murderous impulses particularly toward people who appeared happy, whom he blamed for shaping him into what he had become. Because he has rarely experienced kindness, he finds it confusing and even irritating, struggling to understand genuine compassion when it is directed toward him.

Isaac “Zack” Foster was born into a poor and abusive household and never received formal education, leaving him illiterate. During his childhood, his mother’s lover attempted to kill him by setting him on fire, but Zack survived. Shortly afterward, his mother wrapped his burned body in cloth and abandoned him at an orphanage in exchange for money.

The orphanage served as a front for child trafficking and was run by a man and woman who were physically abusive and neglectful. Food was often withheld for days, forcing Zack to scavenge from garbage to survive. The caretakers referred to him as a “monster” because of his burns and even made him bury the corpse of a dead child, implying this was not the first time he had been tasked with such a grim duty. Though he obeyed without protest, Zack harbored silent resentment.

One day, the caretakers left a television on, and Zack saw a violent horror film that inspired him to take control of his situation. That night, he murdered his sleeping caretakers by repeatedly stabbing them and mutilating their bodies, later burying them in the same yard where he had been forced to bury other victims.

After leaving the orphanage, Zack wandered the streets and encountered his third victim, a woman who had splashed him with her car. He stayed in her vehicle overnight and was discovered the next morning by a blind old man. When Zack held him at knifepoint, the man showed no fear and instead offered him bread. Suspicious but hungry, Zack accepted the food and eventually decided to stay with him.

While living there, Zack spent most of his time sleeping or staring into space. The old man never treated him as a monster, which confused and disturbed Zack, as it contrasted with every other adult he had known. He considered killing the man but ultimately refrained, believing it would be meaningless if the man felt no fear. After killing a drunkard one night, Zack confessed the act to the old man, expecting revulsion. Instead, he was met with a calm question about his future and told to get some rest.

The next day, the old man used his savings to buy better bread for Zack but was attacked and killed by two thugs. When Zack noticed the man’s prolonged absence, he assumed abandonment and destroyed much of the house in anger. Upon overhearing the thugs brag about killing the old man, Zack confronted and murdered them. The satisfaction he felt upon seeing their happiness turn to despair solidified his belief that only “happy” people whom he blamed for creating monsters like himself were worth killing.

Over the following years, Zack continued his killing spree, gaining notoriety as a wanted serial killer and constantly evading the police. Eventually, Reverend Abraham Gray discovered him and offered the position of B6 floor master in an abandoned building designed for his experiments. Granted the freedom to kill anyone who entered without fear of law enforcement, Zack accepted the role without questioning the full scope of Gray’s plans.

Zack is first introduced as the violent and unpredictable master of Floor B6, where Rachel Gardner awakens at the start of Satsuriku no Tenshi (Angels of Death). Initially intent on killing her, he is persuaded to form a pact: in exchange for Rachel’s help navigating the building and its traps, he will kill her once they escape. This unusual alliance becomes the central dynamic of the story.

Throughout their ascent, Zack serves as both Rachel’s protector and a volatile presence, using his physical strength and ruthless combat skills to overcome threats from other floor masters. While he begins the journey driven by his promise and his own violent impulses, Zack gradually develops a deeper connection to Rachel, coming to respect her as more than just a means to an end.

Zack’s impulsive nature often creates tension between them, but his loyalty to their pact and eventually to Rachel herself remains unshaken. His growing emotional investment culminates in moments where he prioritizes her safety and agency over his initial intentions, reflecting significant development in his character by the end of the narrative.

Isaac “Zack” Foster has become one of the most iconic characters from Satsuriku no Tenshi (Angels of Death), standing alongside Rachel Gardner as a central figure in the franchise’s popularity. His distinctive design bandage-wrapped body, scythe, and heterochromatic eyes has made him a recognizable character in anime, manga, and gaming communities.

Zack’s complex relationship with Rachel, shifting from hostility to mutual trust and emotional dependence, is frequently highlighted as one of the series’ strongest elements. Fans often cite his moral code despite his violent nature as a unique trait that differentiates him from many fictional serial killers. His combination of brutality, dark humor, and loyalty has contributed to his enduring appeal.

The character has inspired a wide range of official merchandise, including figures, posters, and apparel, as well as extensive fan art and cosplay. In fandom discussions, Zack is often praised for subverting the “mindless killer” archetype, instead portraying a deeply flawed but emotionally resonant antihero.