Haruki Oguma is introduced as the only son of Fuyuhiko Oguma, the president of the private military company AGS. Despite his father's wishes for him to avoid the danger, Haruki chooses to become an operative of the AQS Blademan unit known as Viper, driven by a need to prove that he can protect Fort Daiva City and its people through his own merit rather than relying on his father's influence. This decision immediately sets him apart from the other, more jaded members of the unit, who view him with skepticism due to his privileged connections. Unlike the mercenaries who prioritize payment and personal safety, Haruki is portrayed as a young man of strong ideals, completely dedicated to the protection of his city and its citizens.

Personality-wise, Haruki is characterized by a potent combination of idealism and naivety. His fervent desire to be a hero and to do what he believes is right often manifests as reckless disobedience. In one of his first missions, he directly ignores a direct order to withdraw, choosing instead to pursue enemy Gellis Bugmechs, an act which forces the veteran Saiki to accompany him to ensure the mission does not end in disaster. This headstrong nature frequently puts him at odds with the more pragmatic members of the Viper unit, as his privileged background as the president's son makes his idealism seem out of touch to soldiers who have survived real wars. Despite this friction, his sincerity gradually earns him the grudging assistance and support of his comrades, such as when Kariya enlists his help to clear Saiki's name.

Central to Haruki's role in the story is his complex web of personal relationships. His primary motivation is a rebellion against his father, Fuyuhiko Oguma, seeking to forge an identity separate from the corporate legacy. The most pivotal relationship is his friendship with Ulla Chiaki, the leader of the anti-PMC terrorist group Hounds. Haruki holds doubts about the official narrative demonizing Hounds and secretly meets with Ulla, believing his friend's claims that the group is not responsible for the bombings they are accused of. This personal connection places Haruki in a precarious position, as his loyalty to a friend clashes directly with his duty as a Blademan and his father's corporate interests.

The trajectory of Haruki's character arc is cut tragically short. As suspicion mounts over a double agent within the Viper unit, Haruki becomes a target. After discovering a hidden garden his father created in memory of his late mother, he is assassinated within the AGS headquarters, with only Saiki and Kariya witnessing his death. His demise is not the end of his significance, however. The AGS corporation cynically exploits his death as a propaganda tool to justify heightened anti-Hound military operations, using his memory to manipulate public sentiment and turn the Viper unit into scapegoats. In a surprising twist at the series' conclusion, Haruki is revealed to be alive, appearing in full health to the public. He assumes control of the company from his father, renames it Daiva Security, and takes over as president, signaling a new direction for the organization he once sought to prove himself independent from.

Regarding his abilities, Haruki is qualified to pilot an AQS Maneuver-Blade, the transforming mecha used by Blademen to fight Bugmechs. His performance in combat, however, is a mix of raw potential and inexperience. He is capable enough to survive engagements and defeat squads of enemy Gellises, especially when guided by a veteran like Saiki. However, his defining trait in battle is not his skill, but his tenacity and willingness to defy orders to engage the enemy, a behavior that speaks more to his personal code of honor than to any strategic or tactical genius.