Akeno Shiranui works as a mermaid examiner tasked with identifying and deporting merfolk who cannot blend undetected into human society. Her strict enforcement of these laws initially makes her hostile and uncompromising towards protagonists Nagasumi Michishio and Sun Seto.

She appears as a pale-skinned teenage girl with dark green hair, often looking black, styled in a ponytail featuring distinctive square-cut bangs. She has matching dark green eyes. Her character design includes a katana, a hime-cut hairstyle, and a school uniform, reflecting her formal position.

Her personality shifts significantly over time; initial antagonism transforms into compassion, especially after interactions with Nagasumi and Sun. This change is partly driven by discovering she is the younger sister of Masa, a member of the Seto Group. Previously unaware of their relationship, she learns Masa once held her examiner role before suffering memory loss. He reconciles with her, becomes protective, and defends her against threats as they overcome their past separation.

Akeno faces personal challenges, including a pronounced fear of ghosts contrasting her stern demeanor. Subtle feelings for Nagasumi also emerge, complicating her interactions. Key episodes show her evolution: she rigorously evaluates Sun's ability to hide her mermaid traits in Episode 16, leading to confrontations. Defeated delinquents follow her, causing Mawari Gōsunkugi to mistake Akeno for a delinquent herself in Episode 17. In Episode 18, her professional curiosity about Sun's physiology creates complications. By Episode 19, she attempts to disrupt Nagasumi and Sun's relationship using a mermaid potion. This plan fails but illustrates her motivations shifting from duty to personal involvement.

Her name, meaning "Venus as the morning star" (明けの明星), connects to celestial and oceanic themes tied to the Shiranui Sea, underscoring her deep merfolk heritage.