Ikoma is the main protagonist of the story, a seventeen-year-old boy who works as a steam smith at the Aragane Station. He has a distinctive appearance, with messy green or yellow-ochre hair, red eyes, and a pair of glasses that feature a lens only over his right eye. Following his transformation, a part of his hair turns white. He also wears a green stone on his right hand, a keepsake from his deceased younger sister, Hatsune.

Haunted by a traumatic event from five years before the story begins, Ikoma's entire worldview is shaped by guilt and a steadfast resolve. When Kabane invaded his hometown, he abandoned his sister in a moment of fear, an act that resulted in her death. This experience forged an iron promise within him to never run away again and to become someone he can be proud of. Unlike most people in his world who live in paralyzing terror of the Kabane, Ikoma is driven by a desire to overcome them through scientific understanding and innovation. He views the Kabane infection not as a curse but as a virus, and dedicates himself to studying it in secret, even using parts of Kabane bodies for his research. This makes him a determined and somewhat unconventional individual, often described as a "badass bookworm" whose courage stems from his intellect and conviction rather than brute force alone. He can be eager to explain his inventions and is a person of strong principles, willing to challenge the rigid, fear-driven rules of his society to do what he believes is right.

Ikoma's primary motivation is twofold: to atone for his past failure by fighting the Kabane effectively, and to protect others from suffering the same fate as his sister. He is not driven by revenge but by a desire to make amends and overcome his own fear. His role in the story is as an agent of change, a lone inventor whose creations challenge the established order and offer a glimmer of hope against the Kabane threat. When his station falls to an attack, he is bitten but uses a self-devised apparatus to choke the infected blood from his brain, successfully preventing his full transformation and becoming a Kabaneri, a half-human, half-Kabane hybrid. After this transformation, he boards the armored locomotive Koutetsujou, where his unique abilities and knowledge make him a pivotal figure in the group's survival.

Over the course of his journey, he forms several key relationships. His closest ally is his childhood friend Takumi, who assists him in his research and inventions. His most significant bond is with Mumei, a young girl who is also a Kabaneri. While she is far more experienced in combat, Ikoma's resolve and different outlook help him connect with her, and he becomes determined to help her escape her violent past and return to being fully human. He also earns the trust of Ayame, the station master's daughter, and Kurusu, a skilled samurai, though this trust is hard-won due to the deep-seated fear and prejudice against Kabaneri. He also comes into conflict with Biba Amatori, a charismatic antagonist whose willingness to sacrifice others for his ideals stands in stark opposition to Ikoma's protective nature.

Ikoma undergoes significant development, beginning as a guilt-ridden, cynical outcast working in the shadows and growing into a more confident and principled leader. His initial cynicism and underlying anger gradually give way to a more steadfast heroism as he earns the respect of those around him. He experiences profound despair, such as after losing his right arm and falling into the sea, only to be inspired by his friends to build a prosthetic replacement and renew his promise to fight. His journey is one of transforming his personal guilt into a selfless drive to protect others, learning to trust and rely on a community rather than acting alone.

As a Kabaneri, Ikoma possesses a range of notable abilities. These include superhuman physical characteristics, enhanced senses, and a mid-low level regenerative capability. His combat skills are augmented by his inventions, most famously the Tsuranuki Zutsu, a powerful, handheld piercing gun that acts as a shaped-charge grenade launcher capable of destroying a Kabane's iron heart-cage. His ingenuity does not stop there; he later creates more powerful jet bullets and even coats his blades in the material from a Kabane's heart cage to increase their effectiveness. Despite his newfound power, Ikoma is far weaker in combat experience compared to other Kabaneri like Mumei, often relying on his strategic thinking and technological innovations to prevail. A significant weakness of his condition is the need to consume human blood; if he runs out, he risks transforming into a full Kabane.