Hiyoshi captains the Vampire Control Division and is the older brother of vampire hunter Ronaldo, despite Ronaldo being physically taller. A former vampire hunter himself, Hiyoshi retired after sustaining an arm injury from a vampire bite. Ronaldo long believed this injury occurred when Hiyoshi shielded him during a high school vampire attack, rendering Hiyoshi unable to fire a gun and forcing his retirement. Hiyoshi later clarified the truth: the injury stemmed from falling victim to a vampire romantic scam, a fact he concealed from Ronaldo.

Though retired from active hunting, Hiyoshi maintains a connection to Ronaldo's work by acting as an intermediary, delivering Ronaldo's manuscript submissions to their publisher, Fukuma. Ronaldo harbors intense admiration and hero worship for Hiyoshi, refusing to acknowledge any flaws and even attributing supernatural abilities like laser vision to him. This dynamic persists even after Hiyoshi's revelation about the true cause of his retirement.

Hiyoshi appears directly in one documented incident where he visits Ronaldo specifically to correct the misconception about his injury. During this encounter, he reassures Ronaldo that the retirement was not Ronaldo's fault, directly addressing Ronaldo's longstanding guilt.