Ruka Azumi • Role: The human girl who is the protagonist of this story (a witness to the world's "Festival"). • Age: 2nd year of middle school (14 years old). • Height: Average for a girl her age (estimated mid-150cm range). • Appearance: Shoulder-length black hair. Features a birthmark (scar) on her arm from a past injury. • Personality: Awkward and struggles to put emotions into words. She has a stubborn, straightforward nature and refuses to back down even when trouble arises in her school club. • Likes: Handball (running), the aquarium where her father works.

Umi • Role: A boy raised by dugongs (the central figure of the "Festival"). • Age: 12 years old. • Height: Slightly shorter than Ruka, with a small build (estimated 140cm range). • Appearance: Sun-kissed brown skin and large, shining eyes. His skin dries out easily on land, often becoming scaly. • Personality: Innocent and cheerful. Highly curious and friendly, he doesn't build walls against anyone. • Likes: Water (the sea or pools), fish, spending time with Ruka.

Sora • Role: A boy raised by dugongs like Umi (Umi's older brother figure). • Age: 13 years old. • Height: About the same as Ruka or slightly taller (estimated 150cm range). • Appearance: Shoulder-length blonde hair with golden eyelashes. His bangs are swept up. Beautiful blue eyes. Translucent pale skin and platinum blonde (white) hair. He has an androgynous, fleetingly beautiful appearance. • Personality: Intelligent and mature, yet somewhat detached. He dislikes being used by humans and is highly wary. • Likes: Anglade the marine biologist, whale songs, the quiet sea at night.

Dede • Role: A mysterious old woman who possesses all knowledge of the sea and the principles of the world. • Height: Small (hunched due to age, appearing even shorter than Ruka and the other children, in the 130cm-140cm range). • Appearance/Features: Deep wrinkles and a piercing gaze that seems to see through everything. She wears unique clothing resembling ethnic attire. • Personality: Quiet and detached, as if she knows the outcome of all phenomena. • Likes: The sea, watching the flow of the world.

Jim • Role: A marine biologist who protects Umi and Sora, living with them while conducting research. • Height: Sturdy, large build (estimated late 170cm range). • Appearance/Features: Sun-tanned, wrinkled skin; a wild-looking old man with characteristic tattoos all over his body. • Personality: Rough and a bit pushy, but possesses the magnanimity to warmly watch over the essence of the sea and the boys' existence. • Likes: Fieldwork, mystical memories of past experiences at sea.

Anglade • Role: A marine biologist with a genius intellect (Jim's former partner). • Height: Quite tall and slender (appears to be nearly 175cm-180cm due to his thin build). • Appearance/Features: Very handsome features with wavy hair left to grow somewhat wildly. He has an androgynous and beautiful appearance. • Personality: Extremely intelligent, possessing a near-obsessive passion for chasing the secrets of Umi and Sora. • Likes: Sora, the truths of the world, "whale songs."

Kanako Azumi • Role: Ruka's mother. She used to work at the same aquarium as Masaaki. • Height: Slightly taller than the average adult woman (around 160cm, as she appears nearly the same height as Masaaki when wearing heels). • Appearance/Features: A woman with a delicate, somewhat sorrowful and mysterious aura. • Personality: Mentally unstable due to a lack of fulfillment in daily life; she is unable to interact well with her daughter Ruka and keeps her distance. • Likes: Memories of the sea from the past.

Masaaki Azumi • Role: Ruka's father. An employee at the Shin-Enokura Aquarium. • Height: Average for an adult male (estimated early to mid-170cm range). • Appearance/Features: A calm, adult man. He has a quiet, awkward presence with a bit of gray in his hair. • Personality: Dedicated to his work with a deep love for the sea and its creatures. Though awkward, he cares for his daughter Ruka. • Likes: Aquarium creatures, quiet time