Kaizen is a monk who appears in the Onimusha anime series. He was born in a temple and has lived his entire life there, unaware that the temple was guarding the legendary Oni Gauntlet.

In terms of personality, Kaizen is dutiful, calm, and spiritually grounded. He adheres strictly to his monastic principles, such as not eating meat, and is often seen praying for the dead. His initial demeanor is pragmatic, even to the point of suggesting that the group abandon a captured girl to avoid a trap, which causes tension with his companions. However, this is rooted not in cruelty but in a focused sense of mission regarding the Gauntlet. Despite this cold practicality, he ultimately displays profound compassion and selflessness, as he is willing to sacrifice himself to save a child.

Kaizen's primary motivation is to fulfill his duty to his master and his temple. He is charged with safeguarding the Oni Gauntlet and accompanying the legendary swordsman Musashi Miyamoto on a mission to stop a rebellion and hunt demons. While his initial role is simply an observer and guardian of the artifact, his journey transforms him. He struggles with the conflict between his monastic teachings of compassion and the brutal reality of their mission, ultimately choosing the path of self-sacrifice.

His role in the story is crucial as the keeper of the Oni Gauntlet, the weapon that allows Musashi to fight demons. He carries the gauntlet in a sealed box and presents it to Musashi when the first demon appears. Beyond this logistical role, Kaizen serves as the group's moral and spiritual compass. He performs funeral rites, offers prayers for the deceased, and is often the voice of contemplation. Most importantly, after he is mortally wounded, he physically stops Musashi from succumbing to the demonic rage of the Oni Gauntlet, reminding him not to become an Oni himself.

Kaizen develops key relationships with several characters. He is subservient and respectful to his unseen master, Unsho. His primary relationship is with Musashi Miyamoto, to whom he is initially an assigned chaperone but later becomes a spiritual anchor, preventing the swordsman from losing his humanity. He also forms a bond with a young village girl named Sayo. He prays with her, comforts her after the loss of her family, and ultimately gives his life to save her from being burned alive under a large bell. In his final moments, he gives her his prayer beads and asks for her not to cry.

Kaizen undergoes a significant development from a passive observer to an active martyr. Initially, he is content to follow orders and remain detached. He admits that he would leave Sayo if necessary to protect the Gauntlet, adhering strictly to his mission parameters. However, when he hears Sayo chanting a mantra under the burning bell, he makes a conscious choice to abandon his mission and save her at the cost of his own life. He burns his hands and face pushing the bell off her, accepting his death without regret, stating that his journey with Musashi was excellent.

Notable abilities possessed by Kaizen include physical stamina and resilience gained from his monk training, which allows him to traverse steep mountain paths with ease while his samurai companions tire. He also demonstrates surprising physical strength, as seen when he pushes a massive, burning bell with his bare hands. His primary strengths, however, are spiritual: he possesses knowledge of sacred mantras, performs funeral rites, and has the wisdom to recognize the corrupting influence of the Oni Gauntlet on Musashi. He does not possess supernatural combat abilities but relies on his faith and mental fortitude.