Jinno, also known later as Kuma, is a major character who begins as a close friend and fellow student of the protagonist, Afro. His life is defined by a profound bond with Afro that tragically transforms into a consuming quest for revenge.

Jinno was an orphan living at a dojo when he and other students discovered a young, severely injured Afro. Jinno called for their master, the Sword Master, who took Afro in. Jinno, along with another student named Otsuru, quickly befriended the new arrival, and Jinno and Afro developed a strong, brotherly rivalry as their skills as swordsmen grew. Jinno favored a unique dual-blade style, allowing him to attack and defend simultaneously, a technique he explained was practical in case he ever lost one arm.

This peaceful childhood ended during the Battle at the Bodhi Tree, a massive assault by assassins seeking the powerful Number Two Headband held by their master. The dojo was destroyed, and most of its students were killed. In the aftermath, Jinno, wounded on the ground, witnessed Afro kill the Sword Master to claim the headband. Consumed by grief and a fierce hatred, Jinno leaped from a cliff in an apparent suicide, despite Afro’s pleas.

However, Jinno survived. He was found by the Empty Seven Clan, a powerful and mysterious organization. Using his profound grief as a focus, the clan's scientist, Dharman, rebuilt Jinno into a cyborg of immense power, replacing his scarred flesh with robotic appendages and a life-support system. Transformed, Jinno donned a new identity as Kuma, wearing a mask fashioned after a teddy bear that belonged to Otsuru. His appearance became a grim spectacle: a black kimono, wooden sandals, and a head made bald with visible robotic plating along his skull. One of his eyes was replaced with a cybernetic implant, while his other eye never stopped crying. He told Afro that his tears had been falling ever since the night of the battle.

In this new form, Jinno’s primary motivation was vengeance. He waited for Afro at the base of the Number One's throne room, tasked by the Empty Seven with blocking his path. Jinno attacked his former friend with relentless fury, blaming Afro for the deaths of everyone they knew. Despite his cyborg enhancements and immense tolerance for pain, he was defeated by Afro, who destroyed his life-support system.

Jinno’s story does not end there. He was rebuilt again by Dharman, though the process cost him his ability to speak. Now working alongside a young woman named Lady Sio, Jinno ambushed Afro at his home, leading to the theft of the Number One headband and setting the events of Afro Samurai: Resurrection in motion. Over the course of this final conflict, Jinno’s character undergoes a final, crucial development. After Afro is killed by a clone, Jinno’s suppressed memories and brotherly love for Afro resurface. Through sheer willpower, he regains his speech and challenges the clone to defend his fallen friend, a valiant but fatal act. He and Sio, who is revealed to be his sister, are both brutally killed. In his dying moments, Jinno reconciles with his sister and passes on. A final spark of electricity from his destroyed cyborg core travels to Afro’s body, resuscitating him and allowing him to defeat the clone. Jinno’s notable abilities include his mastery of the two-sword style, superhuman agility, immense physical strength, and an extraordinary tolerance for pain that allows him to continue fighting even when dismembered or bisected.