Hiroyuki is Genzo Shibata's devoted pet dog, accompanying his owner everywhere—even professional wrestling matches where they execute moves together. Transported alongside Genzo to a fantasy world during a wrestling event, Hiroyuki's species is unrecognized there, leading to his classification as a rare and valuable creature. This status makes him a frequent target for thieves seeking profit.

Despite the recurring kidnapping attempts—primarily orchestrated by the criminal Edgar, who seeks revenge on Genzo and exploits the dog's rarity—Hiroyuki maintains a friendly and approachable demeanor. He readily interacts with strangers regardless of their intentions and exhibits disciplined behavior by consistently using the same spot for bathroom activities. He remains Genzo's constant companion, actively participating alongside him in combat situations. Genzo values Hiroyuki immensely, remarking that the dog would command a significantly higher price than even a vampire maid if sold, underscoring his exceptional perceived value within this world.