Kabamaru, originally named Kagemaru ("absolute shadow"), acquired his nickname "Kabamaru" ("hippo's mouth") because of his enormous appetite and prominent front teeth resembling a hippo's. Orphaned young, he was raised in Iga Province's mountains by his grandfather, the strict ninja master Saizō Igano. Saizō trained Kabamaru and another orphan, Hayate Kirino, rigorously in the ninja arts, enforcing harsh discipline for any incompetence or disobedience. This forged Kabamaru's catlike reflexes, superhuman strength enabling him to carry adults effortlessly, and extraordinary speed allowing him to run 100 km/h and leap skyscrapers. However, his education neglected academics, leaving him functionally illiterate in Japanese and only capable of writing a few borrowed or foreign words.

After Saizō's death, Kabamaru moved to Tokyo under the guardianship of Ran Ookubo, Saizō's former romantic interest and headmistress of Kin'gyoku Academy, where he enrolled. His transition from rural mountain life to urban Tokyo triggered intense culture shock and difficulty adapting to societal norms. His coarse manners, particularly his voracious eating habits—exhibiting "cartoon physics" when devouring food, especially yakisoba noodles and fried shrimp—and lack of decorum initially repulsed Ran's granddaughter, Mai Ookubo, whom he developed an immediate, intense crush on meeting. His country bumpkin persona clashed with the elite school environment.

Kabamaru became unintentionally embroiled in the bitter, decades-long rivalry between Kin'gyoku and Ōgyoku schools, rooted partly in Ran and Ōgyoku's headmistress's past romantic rivalry over Saizō. Kin'gyoku's student council president, Shizune Mejiro, recognized Kabamaru's physical prowess and manipulated him as a pawn against Ōgyoku's leader, Shuu Maejima. Kabamaru participated in schemes like spying on Ōgyoku and competing in the inter-school long-distance relay race. His near-victory there was thwarted by a shuriken attack from Hayate, now working for Ōgyoku. This reignited Kabamaru's desire to reconnect, though Hayate later rejected their childhood bond during a mountain confrontation in Iga, confessing he used Kabamaru's earlier escape attempt as a diversion for his own freedom.

Kabamaru's relationship with Mai evolved significantly. Initially finding him repulsive, Mai gradually acknowledged his kindness and protective nature, especially after he cared for her during an Iga mountain blizzard, sharing an intimate moment where he gave her his beanie. This marked a turning point, leading Mai to eventually reciprocate his feelings. Their relationship faced challenges, including Mai's kidnapping by Ōgyoku forces under Hayate's direction to provoke Kabamaru into fighting. This culminated in a final confrontation where Kabamaru, enraged by Mai's capture, fought Hayate fiercely until Saizō—revealed to have faked his death—intervened. Following this reconciliation, Kabamaru chose to return to Iga with Saizō and Hayate to complete his ninja training, parting ways with Mai after they confessed their feelings and promised to reunite.

Kabamaru later married Mai, and a sequel manga focused on their son, Ko-Kabamaru Igano.