Kino is a young Japanese boy who lives on a mountainside farm with his family. Unlike the villagers below, he views the ocean as a place of beauty and joy where he can swim and play. He is naturally curious, frequently asking his father questions about the world and the differing perspectives between farmers and fishermen.

Jiya is a young boy who lives with his family in a fishing village on the beach. Raised by fishermen, he understands the unpredictable nature of the sea and maintains a deep, ingrained caution toward the water. He frequently warns his friend Kino about the ocean's potential danger.

Kino's father is a hardworking mountain farmer who serves as the primary voice of wisdom for his household. He possesses a calm understanding of the natural world, recognizing both its beauty and its inherent dangers. He teaches his son about the necessity of accepting the unpredictability of life, comparing the nearby volcano to the ocean.

The Old Gentleman is an affluent, elderly man who resides in a grand castle just outside the fishing village. Descending from a historical feudal family, he holds considerable wealth, property, and influence over the local area, including the beautiful Deer Island. He takes his traditional duties seriously, expecting the villagers to rely on his protection during times of natural crisis.

Setsu is Kino's little sister, living on the mountain farm. She is a lively young girl who brings a bright, energetic presence to the household. Often found teasing her brother, she is deeply loved by her family and adds warmth to their daily chores.

Jiya's father is a fisherman who lives with his family in a windowless house on the beach. Having spent his life working on the ocean, he harbors a profound respect and fear of its destructive capabilities. He expects strict obedience from his sons regarding the rules of the water.

Kino's mother is a dedicated farm wife who works alongside her husband and children on the mountain terraces. She provides a nurturing and steady presence in the household, assisting with everything from planting rice to preparing meals.

Jiya's mother lives in the fishing village on the beach alongside her husband and two sons. She shares her family's deeply ingrained respect for the ocean and participates in the daily life of the coastal community.

Jiya's older brother lives and works alongside his family in the coastal fishing village. He learns the fishing trade and the harsh realities of the sea from his father.