Dewey starts as an operative within the antagonistic Dark Joker organization, serving directly under its leader, Buros. He treats battles like competitive games, preferring straightforward confrontations over intricate plots and harboring a bitter rivalry with fellow operative Keto. Following repeated failures against Nurse Angel, Buros modifies Dewey's body to remove his ability to feel pain, resulting in a near-fatal defeat. Nurse Angel intervenes, using her scarce supply of Green Vaccine to save his life instead of letting him perish—an act that fundamentally alters his path.

Seeking to repay this debt, Dewey infiltrates Bross' castle to steal a container of Black Vaccine, informing Nurse Angel she can transform it into the life-saving Green Vaccine. He destroys the container during the mission, returning the purified vaccine to her. This act replenishes her diminishing power source and signifies his official shift in allegiance. Initially distant and awkward in his new role, Dewey slowly accepts the kindness offered by Ririka (Nurse Angel's civilian identity) and her friend Seiya Uzaki. He matures into a dependable combat ally against Dark Joker, engaging in aerial battles and joint missions. His swordsmanship and tactical acumen prove valuable assets, though he maintains a reserved demeanor and initially refers to Ririka formally as "Nurse Angel."

Dewey's accidental spillage of stolen Black Vaccine during the container heist unintentionally fuels ongoing monster outbreaks, necessitating continued battles even after Bross' defeat. His relationships deepen through this conflict: he builds mutual respect with Seiya during collaborative fights and demonstrates protective instincts towards civilians. His emotional barriers completely dissolve in the climax, where he finally uses Ririka's personal name while supporting her during the crisis involving the Flower of Life—marking his full integration into her circle.

Physically, Dewey is a young adult male with blue or green hip-length hair and blue or purple eyes. His attire typically consists of a cape, tunic, and tights, reflecting his otherworldly origin.