Akari Shiodome, originally known as Akari Sakishima, is a significant character from the anime. She is introduced as a young woman of twenty years who resides in the underwater village of Shioshishio, though she regularly travels to the surface world for work. As the older sister of the protagonist Hikari Sakishima, she often finds herself acting as a guiding, if sometimes gently teasing, presence in his life.

Physically, Akari is depicted with chin-length dark brown hair and distinctive ocean-blue eyes, a trait common among the sea people. A small beauty mark under her right eye is a notable feature of her appearance. Her style is casual and cheerful, typically consisting of a light pink top and light blue jeans. Her personality is characterized by a deep-seated kindness, a cheerful disposition, and a nurturing instinct. Following the death of her mother, she took on a mature and responsible role within her family, demonstrating considerable emotional resilience and a selfless dedication to her younger brother's well-being.

A central aspect of Akari's character is her personal journey, which serves as a primary catalyst for the story's initial conflicts. She works at a convenience store on the surface to support her family, a decision that led her to become close with Itaru Shiodome, a surface-dwelling widower and father to a young daughter named Miuna. Her romantic relationship with Itaru is forbidden by the customs of her sea village, which strictly forbids unions between sea and land dwellers due to the fear of their population declining. This conflict forces Akari to make difficult choices, risking banishment from her home and the disapproval of her community to follow her heart.

Her key relationships are the driving force behind her motivations. Her bond with her brother Hikari is strong; she works to ensure he has a future and serves as an emotional anchor for him. Her love for Itaru Shiodome is what compels her to defy her society's oldest traditions. Her relationship with Miuna is particularly complex and poignant. Initially, Miuna resists Akari's presence, fearing abandonment, but Akari's persistent warmth and care eventually help them form a loving mother-daughter bond. After marrying Itaru, she takes the name Shiodome and becomes Miuna's stepmother, later giving birth to a son named Akira.

Throughout the story, Akari undergoes significant development. She begins as a woman torn between her duty to her family and her desire for a life on the surface. Her decision to pursue her own happiness is a courageous act against societal pressure. After a major event that leads to a five-year time skip, Akari has fully embraced her life on land. Her appearance matures, with her hair grown longer and tied back, and she is often seen wearing a seashell necklace given to her by Miuna, symbolizing their unbreakable family bond. She navigates the guilt and sorrow of losing contact with her underwater family and friends, yet remains a pillar of strength and compassion for those around her. As a resident of the sea village, she possesses an ena, a special layer that allows her to breathe underwater, a trait she shares with her people.