Kappei is a young boy who originates from Oedo and is one of the main characters within his world. His background is deeply rooted in tradition, as he lived with and trained in swordsmanship under his grandfather, Gen'unsai. It was his grandfather who revealed to him that he is a descendant of a long line of shinobi. Kappei's family has held a sacred duty for generations: to guard and protect an extraterrestrial, gum-like being named Gumchi from those who would seek to exploit his power. After a decisive battle at his family's dojo where he first used Gumchi's power to defend himself, Kappei was sent by his grandfather to Komerika. His mission there was to train under his great-uncle, Genryusai, and to master the ways of the modern ninja while continuing to keep Gumchi safe.

In terms of personality, Kappei is best described as a hot-blooded yet innocent and carefree boy. He possesses a strong will and is fiercely protective of those he cares about, willing to take on any foe to defend his beliefs. While he takes his ninja studies, including reviewing scrolls and completing missions, with great seriousness and diligence, he is often found napping or goofing off when not engaged in training. His communication style can be a bit slow, and he tends to be straightforward, sometimes reporting important information long after others are already aware of it. He has a noted extreme sweet tooth and a preference for traditional loincloths over modern underwear.

Kappei’s primary motivation is to become a strong ninja and to honor his family’s legacy, all while protecting his companion Gumchi. Within the story, he acts as a guardian and best friend to Gumchi, with the two sharing a bond that is central to his identity. He also acts as a support to other characters, such as when he accompanied Berecca during her mission to Oedo, keeping an eye on her throughout her time there. His relationships are key to his development; he forms a close, though often competitive, bond with fellow ninja-in-training Van, with the two sharing similar mannerisms such as falling asleep during long lectures. Throughout the animated series, Kappei participates in numerous tournaments and battles, including a championship where he attempted to use stealth techniques, and a decisive one-on-one fight against a rival named Allen. He is shown to be a capable fighter, but he also faces defeats and challenges that contribute to his growth, such as losing in a student council election and learning hard truths about his mentors.

Kappei has a distinctive appearance that matches his energetic personality. He has spiky light blue hair that sticks through a backward baseball cap. He wears a dark blue hoodie, often associated with the in-game brand XINOBI, and puffy denim pants with a blue camo pattern. His design typically includes a small X-shaped scar on his left cheek and a bandage on his nose. As with the other main characters, Kappei has consumed Ninja-gum, which allows him to weaponize S-Energy in combat. His signature weapon of choice is the Drill Beast, a weapon classified similarly to a Japanese sword. His associated shinobi powers are primarily swordsmanship and the use of stealth techniques.