Jiro Kusano, universally nicknamed Roji, serves as deuteragonist and primary assistant to Toru Muhyo in their supernatural investigation bureau. His path to magic law began in childhood when a friend’s ghost possession was resolved by executor Page Klaus, inspiring Roji’s career pursuit. After failing the practitioner academy entrance exam, he accidentally interviewed for Muhyo’s assistant role, drawn by a "No magic law experience needed" sign. During an awkward interview, his latent potential surfaced upon instinctively injuring a grudge spirit with a staff, securing his position under Muhyo.

Roji embodies profound empathy and optimism, persistently believing in others’ goodness—though this idealism sometimes enables manipulation by malevolent spirits. Sensitive and prone to anxious, tearful outbursts that contrast with Muhyo’s sternness, he nonetheless displays unwavering courage, often risking himself to protect others. His kindness extends to adoring cute objects, tea, sweets, and crafting intricate costumes/gadgets via "Super-Fab-Tech," reflecting engineering, mathematics, and physics proficiency. He dislikes ghosts and Muhyo’s intimidation tactics, wears contact lenses for near-sightedness, and suffers severe motion sickness. Roji’s emotional openness is occasionally perceived as feminine, and he harbors a noted fondness for Muhyo, finding him "adorable."

Initially functioning as a second clerk with minimal combat skills, Roji relies on a magic pen to create binding seals for defense or support. His early ability, "Direct Binding," immobilizes ghosts via inscribed seals. His power fluctuates with confidence, revealing hidden potential in crises—such as wielding tools typically reserved for higher-ranked practitioners. A pivotal exam uncovered his "extraordinarily high tempering" (spiritual energy). Page Klaus later revealed gifting Roji a bolo tie to suppress this volatile tempering, which inadvertently attracted spirits. Roji’s growth culminates in his role as a "Spanner in the Works" against antagonists like Teeki, disrupting schemes including Teeki’s attempt to possess Enchu.

Roji’s relationships define his emotional core. His bond with Muhyo evolves from perceived disdain to mutual reliance; Muhyo secretly values Roji’s honesty and selflessness, having chosen him despite lacking qualifications. Interactions with ally Reiko Imai carry romantic undertones from her side, while Roji’s ambiguous attractions—to Nana Takenouchi, Rio Kurotori, Enchu, and Muhyo—suggest potential bisexuality. His backstory includes a grandmother mentioned in drama CDs. Roji’s unwavering loyalty positions him as Muhyo’s moral anchor, reinforced through shared experiences like studying "Jabin" texts.