Fujio Saito, universally known as Saru ("monkey"), sports short black hair and brown eyes, consistently dressed in a black button-up shirt layered over white. He attended school with Makoto and nurses a deep-seated grudge over unpaid debts dating back to their eighth-grade years. To shed his self-perceived weakness, Saru joined a local yakuza family. His inaugural assignment—protecting the boss's daughter—ended in failure, compelling the yakuza to enlist Makoto's help. As punishment for this lapse, Saru endured the traditional yakuza finger-cutting ritual, losing his pinky.

Post-incident, Saru operates throughout the series as Makoto's fluctuating ally and underworld conduit, exploiting yakuza ties to broker information or favors. Their exchanges perpetually simmer with Saru's entrenched resentment toward Makoto, yet pivot on grudging dependence born of Makoto's famed problem-solving prowess in Ikebukuro.