Yuma Isogai appears in the "Koro Sensei Quest!" spin-off as a member of Class 3-E within a fantasy RPG setting. He retains core traits from the original series, adapted to this comedic fantasy universe. Isogai wears distinctive blue-and-white hero armor, setting him apart visually from classmates in standard attire. However, a character-specific bug causes any armor or clothing he wears to vanish from the back half, leaving his back and underwear exposed. This effect extends to environmental objects like bushes when he attempts to hide. The bug, described as an irregularity tied to his character, is attributed to perpetual poverty limiting him to half a whole-body armor set, secured only by rope ties at the neck and waist. Despite this exposure, his classmates do not consider him a pervert.

The bug creates recurring comedic situations. For instance, when Class E tries to enter a realm protected by a purity barrier repelling impure thoughts, Isogai cannot pass due to his involuntary state, alongside other male classmates deemed mentally impure. This contrasts with his otherwise heroic appearance and demeanor. His background as a financially struggling student persists; his father died in a traffic accident during his first year of junior high, leaving him to support his mother and younger siblings, referenced in dialogue about his partial armor affordability. Personality-wise, he remains a reliable leader and "ikemen" (handsome with a flawless personality), coordinating classmates during quests with consistent kindness and tactical acumen. His role leverages these original traits for humor through the visual gag of his bug, while his leadership and popularity remain intact.