Hiroshi Kozumi is a central figure in the story, depicted as the fiance of a woman named Michiko. He works alongside her at a pharmaceutical company, where his role involves conducting unusual and dangerous experiments on mice, which result in the animals mutating into monsters before exploding.

Hiroshi's motivations are driven by a secretive agenda. He persuades Michiko to help him steal a classified file from the company's vault, claiming the action is necessary for their future together. The file is revealed to contain data on something called "Project Gagou," which includes information on The Skull Man's armor. This suggests a deeper, possibly self-serving ambition, as he communicates with an unnamed contact to arrange a pickup of the stolen documents.

His relationship with Michiko is a key element of the story. While they are engaged, Hiroshi involves her in his dangerous scheme by having her disable security systems. Ultimately, his actions lead to a violent confrontation. After securing the documents, he and Michiko are intercepted by armed men at a pre-arranged drop point. As Hiroshi attempts to surrender, he is shot through the heart by a sniper and killed.

Before dying, Hiroshi performs a final, significant act. He pulls a pendant from Michiko's neck. This action, combined with the grief of witnessing her fiance's death, triggers a monstrous transformation in her. She turns into a lizard-like creature and goes on a rampage. In her dying moments, she has a vision of Hiroshi, who tells her that her nightmares are finally over. His role in the story is therefore a catalyst, his death and final action directly causing the pivotal transformation of Michiko. Apart from his scientific experimentation and his role in the theft of the "Project Gagou" documents, no other notable abilities are demonstrated.