Aoto is a sixteen-year-old boy who serves as one of the main protagonists. He is known by the infamous nickname the Parent Killer, a rumor that has led his peers to shun him and isolate him from others. In truth, Aoto endured a miserable and loveless childhood. His parents treated him harshly, favoring his younger twin brother, Ariton, while Aoto himself was often kept at a distance, eating cold food and living apart from the family. One night, Ariton killed both of their parents and fled, leaving Aoto as the sole survivor. Rather than reveal the truth about his brother, Aoto shouldered the blame for the crime, which is how he earned his dark reputation. His motivation stems from this traumatic event; he wishes to find his brother again to understand the truth about the night of the murder.

Aoto is calm, cold, and collected, rarely showing any signs of emotion. He is naturally reserved, often keeping to himself and maintaining a wall around his heart. This demeanor is reflected in small habits, such as his refusal to eat warm food, which symbolizes his inability to accept emotional warmth. Despite this cold exterior, he is not without a sense of responsibility or pragmatism. He is a skilled swordsman who is adept at finding an opponent’s weaknesses, and he can be practical enough to take down a building if it means defeating a target. Inside his mind, he is haunted by a subconscious manifestation of himself as a little boy who constantly reminds him of his misery, representing the internal rain of his tears that never seems to stop falling.

At the start of the story, Aoto already possesses considerable power as a water user and can see the Divine Gate, marking him as a Transferer. Unlike others who need a special focus item known as a Driver to use their abilities, Aoto is powerful enough to manipulate water on his own, even forming it into a katana for combat. He has little interest in using his power, believing it has never done him any good. He is approached by Arthur, the leader of the World Council, who wishes to recruit him for the quest to find the Divine Gate. Arthur also gives Aoto his own Driver, a blue katana named Wadatsumi, which further focuses his abilities.

Initially, Aoto refuses to join the academy or Arthur’s cause, preferring his isolated life. However, after encounters with fellow Adapters Akane and Midori, as well as being manipulated into a crisis by the god Loki, Aoto begins to reconsider. He decides to join the academy, partly because he wants to see his brother again and partly because he finds Akane and Midori to be annoyingly persistent. His role in the story becomes that of a reluctant hero whose journey is one of learning to move forward despite his trauma. Throughout the series, he forms a key relationship with Akane and Midori, who become his teammates in the Knights of Round, Arthur’s elite group. Their persistence and different perspectives slowly help him learn to open up. His relationship with his brother Ariton remains the core of his personal conflict, as Ariton represents the past that Aoto must confront. Aoto’s development is defined by his gradual shift from a boy paralyzed by guilt and sorrow to one who can accept his past and work alongside others to face the future. His abilities include superhuman physical attributes, water manipulation, and the creation of barriers for defense.