Fumio Fukamachi, father of Sho Fukamachi, raised his son alone after his wife died when Sho was around three. He worked as a salaryman. Chronos Corporation abducted Fumio, taking him hostage to their Mount Minakami base. There, scientist Dr. Hamilcar Barcas forcibly transformed Fumio via Zoanoid processing into a specialized anti-Guyver weapon, Enzyme II, also called the Guyver-Killer. Chronos exploited Fumio's connection to Sho, believing Sho would hesitate to fight his own father. After optimization, Fumio was held until Sho rescued him.

During their escape, Fumio transformed into Enzyme II. Under Dr. Barcas's telepathic control, he attacked Sho. Overwhelmed by confronting his transformed father, Sho could not fight back effectively. Enzyme II crushed the Guyver armor's head and ripped out part of its brain module, inflicting near-fatal damage. This triggered the Guyver armor's autonomous defense system. It overrode Sho's incapacitated state, severing Enzyme II's limbs and destroying the creature using the Mega-Smasher weapon. As the Mega-Smasher struck, Fumio briefly regained consciousness, recognized Sho, and screamed his son's name before being obliterated.

Fumio's death traumatized Sho profoundly. The trauma of killing his father caused Sho to subconsciously repress the memory, creating a mental block preventing his Guyver transformation. Sho only regained this ability after encountering the rogue Zoanoid Aptom, who revealed the truth about Fumio's fate, forcing Sho to confront the event. Fumio's transformation and death became a catalyst for Sho's resolve to battle Chronos.