Akuto Sai is the central protagonist of his narrative, a fifteen-year-old orphan whose life takes an unexpected turn upon enrolling at Constant Magical Academy. His background is marked by abandonment, as he was left on the steps of a church as an infant and raised there, fostering a deep-seated desire to contribute positively to the world. Driven by this goal, he enters the academy with the specific ambition of becoming a high priest, a role of great moral authority and service. However, on his very first day, a school oracle known as Yatagarasu, which predicts each student's future, proclaims that Akuto is destined to become the next Demon King, a title associated with tyranny and destruction.

In terms of personality, Akuto is fundamentally a kind and good-natured individual with a powerful sense of justice. He consistently tries to use his abilities for noble purposes and resents the label of "Demon King," persistently attempting to prove that he is not a bad person. His behavior is often chivalrous, and he tends to be uncomfortable with the unwanted romantic advances he receives. A recurring theme in his life is a struggle with public perception; his every sincere attempt to demonstrate his goodness is frequently misinterpreted by others as a sinister act, cementing his undeserved reputation as a villain. This dynamic often leads to him being an accidental pervert in various comedic situations, further complicating his social standing. He can also be blunt and lacking in social graces, sometimes saying thoughtless things that hurt the feelings of those close to him without intending to.

Akuto's primary motivation evolves over the course of his journey. Initially, he is driven by a simple wish to become a high priest to help people and make the world a better place, a goal he shares with fellow student Junko Hattori. After being labeled the Demon King, his motivation shifts towards a desperate struggle to defy what appears to be a predetermined destiny. He seeks to prove the prophecy wrong and show that he can choose his own path. This later expands into a larger rebellion against a societal system that forces individuals into fixed roles based on magical aptitude and prophecy, aiming to protect people's freedom to choose their own futures. His role in the story is that of an unwilling and misunderstood anti-hero. He becomes a magnet for conflict, as the school and much of society view him as a threat, forcing him into a position where his powerful abilities are often used in self-defense or to protect others, which ironically only fuels the fear surrounding him.

His key relationships are complex and form a core part of his experience. Junko Hattori is a descendant of a ninja family with whom he forms a friendship pact. She has a conflicted "love-hate" relationship with him, torn between her growing feelings and her duty to fight evil, which she believes him to be. Hiroshi Miwa is a devoted fanboy who, after being saved by Akuto, becomes like a self-styled younger brother to him, offering unwavering loyalty. Keena Soga is a fellow orphan from his childhood church who reappears at the academy; she is a constant, eccentric friend who hovers around him, hoping for his success regardless of his title. Korone is a robotic observer assigned to monitor him, while Fujiko Eto is a black mage who actively seeks to become his "dark wife" and help him embrace his demonic destiny.

Throughout his development, Akuto transitions from a naive and hopeful boy to someone who accepts the burden of his immense power, not for evil, but to challenge the very system that condemns him. He learns that fighting against his fate is not about avoiding power, but about how he chooses to wield it. His notable abilities are vast and often destructive, marking him as a person of mass destruction. He possesses an enormous amount of raw magical energy called mana, which he initially struggles to control. This power grants him superhuman physical strength, the ability to create massive explosions, and a healing factor that allows him to recover from serious injuries. He can also absorb and nullify demonic energy from other beings, and when his powers are active, his eyes glow red and red markings appear below them.