Akira Nikaidō is a sixteen-year-old high school student and the central protagonist of his story. He is a second-year student at Private Kiriha High School. Physically, he stands 175 centimeters tall and has a slim, well-balanced build that prioritizes speed and agility over raw endurance. In his everyday human form, he has naturally light brown, neck-length hair and grayish eyes. He typically wears his school uniform or casual clothes like tight-fitting shirts and black jeans, often accessorizing with a choker and a small hoop earring. He has a habit of listening to loud music, which has contributed to some hearing loss. Despite frequently skipping classes, he is quite intelligent, particularly enjoys English, and is talented in sports and playing musical instruments like the guitar and piano.

Akira possesses a personality that is outwardly aloof, blunt, and quick-tempered. He is known for getting into fights, though he rarely starts them himself, often being targeted by bullies or delinquents because of his standoffish demeanor. He finds school boring and is a habitual truant, preferring to nap on the rooftop or pass the time with video games. Despite his cynical and sometimes morbid sense of humor, this rough exterior hides a surprisingly considerate and kind nature. He is a classic tsundere, often acting harshly to conceal his genuine care for others, such as when he tries to push his friend Kengo away to keep him out of danger. While he appears apathetic, he can be roused to deliver wildly exaggerated and surprisingly effective motivational speeches.

His motivation to act is largely reluctant at first. He is forced into his role as a protector when a mysterious silver-haired man named Shirogane appears and reveals that the balance between the human world and a parallel shadow world has been distorted. After being attacked by shadow monsters known as Kokuchi, Akira is left with no choice but to contract with Shirogane to survive. This transformation allows him to fight, but it also forces him to confront a destiny he never asked for. As the story progresses, his motivation shifts from mere survival to a more personal stake when he learns he is the reincarnation of Ryuko, a powerful king of the light realm called Rei, making him a direct descendant of royal lineage.

Within the story, Akira serves as the primary fighter against the forces of darkness. As a Shin, a creature of the shadow world, he loses his human shadow and must rely on Shirogane or a special object called a doppler to remain visible to normal people. His role evolves from a reluctant participant to a key figure in the struggle to restore balance, ultimately accepting his heritage as Ryuko's successor.

Akira's key relationships are central to his character. His bond with Kengo Asamura is his most significant friendship. Despite Akira constantly berating him and treating him like a servant, Kengo remains an optimistic and loyal companion, and their relationship is one of deep, unspoken trust. Aya Suzuno, a member of the disciplinary committee, often drags Akira to class and cares for him on a friendly level, though she is harsh with him. The relationship with Shirogane is complex, beginning with distrust and reluctance as Shirogane drags him into this new world. Over time, they develop a tense bond as mentor and student, with Shirogane keeping many secrets about Akira's past life. Akira also shares a close, sibling-like bond with Kou, an older figure whom he calls Kou-nii.

Throughout the narrative, Akira undergoes significant development. Initially, he has no memory of his past life as Ryuko and is completely unaware of his true power. He experiences at least one incomplete awakening in the past, a traumatic event that was erased from his memory. As he fights and grows, he gradually regains these memories and awakens to his full potential. His development culminates in him defeating his adversaries and accepting Ryuko's inshi, the core of his predecessor's power and memories, allowing him to become the new Direct King of Rei.

As a human, Akira is a capable fighter with excellent reflexes and a high tolerance for pain, though he lacks stamina. When he transforms into a Shin, his hair turns jet black and his eyes bright red. In this state, his physical abilities are superhumanly enhanced; he can jump from great heights without injury, run faster, and even fly. His primary weapons are a pair of hunting knives with blades that are transparent like glass. These weapons are a direct outgrowth of his will to fight. As he grows stronger, these daggers evolve into larger, more complex sets of blades. After fully awakening as the Rei King, he wields a single large sword and gains the ability to control ice and cold currents, as well as manipulate water.