Haruki Takehaya is a pivotal character from the post-apocalyptic narrative of Heavenly Delusion, whose identity becomes deeply intertwined with the series' central mystery. He is initially presented as a young boy living with his older sister, Kiriko Takehaya, and other orphans at a facility known as the Funayama Orphanage, where they are cared for by a man named Robin Inazaki. Haruki looks up to Robin, who leads a group that fights the monstrous creatures known as man-eaters, and is eager to prove his own worth and help protect those around him.

In terms of personality, Haruki is characterized by a certain impulsiveness and a reckless drive to take action, often without fully considering the dangers involved. This tendency is fueled by a deep, almost obsessive devotion to his sister, Kiriko, whose approval and safety are central to his motivations. He desperately wants to move from being the one who is protected to being a capable protector himself, which leads him to take dangerous risks.

Haruki’s role in the story, though brief in his original form, is the catalyst for one of the main characters. His defining moment occurs when, during one of Kiriko’s electro-kart races, he spots an invisible man-eater on the track. Unable to stop the race, he decides to confront the monster alone. He is quickly overpowered and nearly consumed, losing the lower half of his body in the attack. His sister rushes to save him, pulling his remaining upper body from the creature, but she is then shot in the head by an unknown assailant. On the verge of death, both siblings are operated on by a doctor named Sawatari. As Kiriko is brain-dead from her wound, the doctor performs a radical and ethically ambiguous procedure, transplanting Haruki’s intact brain into his sister’s healthy body. This operation effectively erases Haruki as a distinct individual and gives rise to the character Kiruko, who possesses Haruki’s memories and consciousness within Kiriko’s form.

Key relationships define Haruki’s brief existence. His relationship with his sister, Kiriko, is the most significant, marked by his fierce protectiveness and emotional dependence on her. Robin Inazaki also serves as a crucial figure, acting as a guardian and mentor whom Haruki admires and seeks to emulate. This admiration later becomes a source of deep trauma for Kiruko when Robin’s true, predatory nature is revealed. The doctor, Sawatari, is another pivotal figure, not through a personal bond, but through his actions that fundamentally alter Haruki’s existence.

The development of Haruki is less about change and more about the tragic conclusion of his arc, which forces him into an unimaginable new reality. After waking up as Kiruko, he experiences a profound identity crisis, having to navigate the world in his sister’s body while carrying his own memories. He adopts the name "Kiruko," a blend of his and his sister's names, and continues on, driven by new motivations: to find Dr. Sawatari for answers and to locate Robin Inazaki. This new identity becomes a core part of the story, as Kiruko grapples with the interplay of his male consciousness and her female body, eventually working as a bodyguard and traveling with the protagonist, Maru.

As a boy, Haruki possessed a few notable abilities. He demonstrated skill with ranged weapons, crafting his own crossbow and showing confidence and accuracy with firearms. He was also a member of a racing league, which gave him quick reflexes and agility. However, his most significant abilities are those that continue on in his new form as Kiruko, primarily his proficiency with a powerful pistol, which becomes his main weapon against the man-eaters that plague their world.