Hyōka Midarezaki, designated "The Black Thirteenth," is the youngest scion of the Midarezaki family—a three-year-old biological weapon forged from the flesh of Enka, the god of destruction. Engineered as a vessel of annihilation, he defied his purpose by obliterating the laboratory that created him, erasing his own memories of origin and violence to escape his past.

Hyōka’s physiology diverges from humanity, lacking taste buds or a digestive system, which allows him to consume meals prepared by Kyōka Midarezaki unharmed—a unique trait among his kin. His existence intertwines with Dekamelon, a green-hued sibling entity mirroring his appearance, though their interactions skew comedic, often derailing Dekamelon’s attempts at formal self-introduction.

A pivotal tragedy shapes Hyōka’s history: during a test orchestrated by his creator, Dr. Gebok, he unwittingly killed Takanashi Yuiko, the sole friend he cherished. Gebok strategically positioned Yuiko in his path to evaluate his violent potential, an act that ignited Hyōka’s rebellion and flight. The memory of this trauma lay dormant until reuniting with Kiriko Takanashi, Yuiko’s sister.

Hyōka’s emotional growth orbits his bond with Kiriko, which deepens into mutual affection. Kiriko delays formalizing their relationship until resolving her own identity struggles. Despite his programming for destruction, Hyōka gravitates toward pacifism, a contradiction that fuels his struggle to reconcile his weaponized nature with his place in a familial structure.

Dr. Gebok, facing scant consequences for his manipulations, persists as a recurring antagonist. Hyōka’s journey within the Midarezaki family underscores his tentative steps into human social frameworks, his non-human biology and engineered origins perpetually shaping his path as he navigates the aftermath of his actions and the duality of his existence.