Kaimaru is a young boy who resides with the group of humans banished to Hinoe Island alongside the harvest goddess Sakuna. His background is marked by tragedy; he is the son of a bandit leader who was murdered by the antagonist Ishimaru. Rescued by the farmer Tauemon following this traumatic event, Kaimaru was brought to the Lofty Realm and later accompanied the others into exile. The shock of witnessing his father's death caused Kaimaru to lose the ability to speak, rendering him effectively mute and contributing to a disposition that is notably gentle, carefree, and somewhat infantile compared to his peers. Despite this childlike demeanor, he possesses a profoundly kind heart.

Kaimaru's most significant and defining trait is his unique ability to communicate with animals and beasts. This gift allows him to hold entire conversations with creatures such as dogs, cats, ducks, and even a cow, making him a fast friend to all manner of wildlife. He takes on the crucial role of caring for the animals at the farm pen, and his efforts directly aid Sakuna and the group by making life on the island easier. For example, the animals he befriends can be dispatched on errands, and the cow he finds can be used with a plow to till the rice fields efficiently. Additionally, when sent to gather resources, Kaimaru focuses on collecting items specifically used for creating fertilizer, contributing to the success of the rice cultivation that is central to the community's survival.

In the story, Kaimaru's personal journey involves moving from a state of vulnerable dependency to one of resilient strength. Although Ishimaru once attempted to abduct him, Kaimaru awakened nascent divine powers to fight back and fend off his father's killer, demonstrating a latent inner power. His spiritual abilities extend further; he can project his soul outside his body while in a sleeping state. He uses this power to follow Sakuna during her final battle with the great serpent deity Omizuchi. After being swallowed by the creature, Kaimaru’s soul is released upon Omizuchi's defeat, and he provides crucial motivation to help rescue Sakuna. It is during this pivotal moment that he finally regains his ability to speak.

Kaimaru's key relationships shape his life within the group. He receives maternal care from Myrthe and paternal guardianship from Tauemon, both of whom treat him like a son. Sakuna is generally caring toward him, and he gets along well with the Kappa, a friendly monster. However, his behavior can strain his relationships with other humans; Yui is often annoyed by what she perceives as his clumsiness and will sometimes hit him on the head, while Kinta does not click well with him and is easily irritated by his antics. Following the final defeat of Omizuchi, Kaimaru returns to the Lowly Realm with Myrthe and Kinta. There, his deep empathy for others allows him to grow into a good farmer and, ultimately, a counselor for his town. His name reflects his dual nature, combining a meaning related to raising animals with a traditional suffix meant to ward off calamity.