Hiroki Honma is the brother of Natsume Honma and son of Kumi Honma. He crafts a supportive facade, presenting himself as the object of Aya Iseshima's romantic interest to manipulate and gain her trust for a larger scheme.

Hiroki actively conspires with his sister Natsume and their cousin Miko Kazuki to orchestrate the Martial Hearts tournament. Their goal is vengeance against Aya's mother, Suzuko Iseshima, for her role in a prior tournament that resulted in Kumi's sexual slavery. Hiroki manipulates the tournament outcomes, ensuring defeated competitors face enslavement, mirroring his mother's fate.

In the final confrontation, Hiroki, alongside Natsume and Miko, reveals their true motivations and betrayal to Aya. He directly participates in the attempt to subjugate Aya, intending her enslavement. Hiroki also kills Izumi Hayakawa, a co-conspirator attempting to betray their group.

His actions culminate in a violent encounter with Suzuko, who intervenes to save Aya. Suzuko confronts Hiroki, Natsume, and Miko, revealing the cycle of revenge stemming from their family's actions against hers. She kills Hiroki by snapping his neck during their attack. His death occurs alongside Natsume and Miko in a burning building.

Hiroki's background is intrinsically linked to Kumi's history. Kumi was rendered mute after losing the prior Martial Hearts tournament to Suzuko, leading to her enslavement. This trauma motivated Kumi to groom Hiroki and Natsume into executing the vengeance against Suzuko and her family. Hiroki's participation stems directly from this familial manipulation and the legacy of violence.

Hiroki remains unwavering in his commitment to the vengeance plot until his demise. His narrative function centers on perpetuating the cycle targeting Aya and Suzuko.