Toi Kuji is a fourteen-year-old male with striking blue hair that often falls over his right eye, vivid blue eyes, and scattered facial freckles. His typical outfit consists of a black hoodie worn over an open white shirt, a red tie, blue trousers held by a brown belt, and sturdy navy boots. When transformed into a kappa, his skin turns turquoise blue while retaining his hairstyle; his eyes become solid black, a sara plate sits atop his head, a teal neckerchief adorns his neck, and his shell bears a lightning bolt design.

Born on November 1, Toi endured childhood trauma at age ten when his parents died by suicide under crushing debt. He then lived with his older brother Chikai, who ran the family soba shop while engaging in crime for survival. Chikai taught Toi that "bad guys" endure in this world, leading Toi to vow to stay by his brother’s side. This commitment hardened after Toi shot a yakuza member targeting Chikai in self-defense, cementing his place in his brother’s criminal world.

As a middle school transfer student in Kazuki Yasaka and Enta Jinnai’s class, Toi participates in delinquent acts like car break-ins and selling cannabis for Chikai. He displays a readiness for violence—including waterboarding and firearm use—reflecting his tough exterior, yet shows fierce loyalty, especially toward Chikai. Raised in the soba shop, he loves soba but despises pickled plums.

His kappa transformation triggers after he shatters a kappa statue during a fight with Kazuki. Tasked by Keppi, prince of the Kappa Kingdom, Toi reluctantly teams with Kazuki and Enta to collect "Dishes of Hope" by defeating kappa zombies and extracting their shirikodama. His initial "shoot first" mentality softens after finding Enta injured and questioning Enta’s loyalty to Kazuki, which forces introspection about his own choices.

A pivotal blow strikes when Chikai declares he won’t return to Asakusa, leaving Toi shattered. Kazuki soon gives Toi his own Dish of Hope, claiming he "hates" his brother Haruka, though Toi’s reaction goes unrecorded. Toi later aids Kazuki in kidnapping idol Sara Azuma, further entangling their paths. In the finale, Toi reunites with Chikai but is separated from Asakusa. Three years post-juvenile detention, he returns to Asakusa, reflecting: "This world is filled with connections... But everything in my life is done. What’s lost is never coming back." He shouts "So, what’s wrong with that?!" before leaping into a river—where Kazuki and Enta surprise him by following.

Toi blends impulsivity and pragmatic criminality with buried vulnerability. He struggles to voice affection but offers forgiveness to those he cherishes. His defining quotes—"People realize they were connected when they no longer are" and his final river monologue—anchor his narrative on the fragility of bonds.