Daisuke Doujima is a second-year student at Seisho Academy. In his childhood, he was kidnapped and then rescued by a mysterious woman named Milo, who delivered a prophecy: he would be visited by a terrible danger and would become the only one capable of protecting everyone. He devoted his life to preparing for this foretold moment, a commitment that isolated him from his peers, though he never wavered in his belief.

This prophecy fostered an extreme hero complex, resulting in persistent overreactions to minor incidents and a stubborn, determined nature. He is hot-blooded, rash, and prone to violent outbursts, having assaulted a cyclist who bumped into his friend and broken a stranger's ribs for asking for directions. He remains oblivious to the romantic feelings of others.

He has black hair and grey eyes and is slightly shorter than his friends Gai and Keisaku. He carries a balisong in his pocket.

When the city of Shibuya is transported over 300 years into a future wasteland, he is reunited with Milo, who does not initially recognize him. She provides him and his childhood friends with mechanized weapons called String Puppets, which they are uniquely able to pilot. Daisuke readily accepts the role of protector, seeing it as the fulfillment of his long-awaited destiny.

As a String Puppet pilot, he shows immediate proficiency and undergoes months of training to hone his skills. His unit is equipped with an AI for situational analysis and enemy assessment, along with weaponry including missiles and homing attacks. It enables flight and interaction with non-physical entities known as the Gespenst, ghosts of an ultra-high dimensional nature existing outside time. The String Puppet also has heat signature detection capabilities.

His combat effectiveness is rated at Wall level, allowing him to cause and withstand destruction comparable to a wall's durability. He possesses superhuman lifting strength, and his equipment is powered by internal batteries that last for months.

The death of his best friend, Keisaku Asano, becomes a significant turning point. It shatters his hero complex by confronting him with the failure of his protective motto. He shifts his focus toward the goal of returning Shibuya to its original time, maturing past his reckless and self-centered behavior to become a more genuine and sympathetic leader.

His relationships with his childhood friends are complex. While driven to protect them, his actions often strain their bonds. Gai Steiner frequently challenges his authority as a contrarian commander, creating conflict. Keisaku had acted as the group's heart, often mediating between Daisuke and the others, and his death impacts everyone deeply. Marin Temari holds romantic feelings for him, which he does not notice.

During their time in the future, he learns the prophecy may have been misinterpreted. It is revealed that the organization AHRV originally foresaw Keisaku as the most important person destined to pilot the String Puppet, with Daisuke considered only a backup. Milo disobeyed orders and chose Daisuke to protect everyone, altering destiny.

His journey involves battling the Revisions, entities that harvest humans for parts to prolong their own lives. The leader of the Revisions, Nicholas, eventually merges with Keisaku's memories and body to become an immortal being existing across all time. In the final conflict, Keisaku's consciousness sacrifices itself to stop Nicholas, enabling Daisuke and Shibuya to return to their original time.

After returning, he declares his intention to find and rescue Keisaku, channeling his determination into a more focused and less egotistical mission.