Shiranui Akeno serves as a mermaid examiner enforcing secrecy laws among merfolk, primarily deporting those who risk exposure. She initially monitors the Seto group, focusing on preventing San from revealing her nature and interfering with San's marriage to Nagasumi Michishio.

Akeno possesses pale skin, dark green hair typically tied in a ponytail with square bangs, and matching dark green eyes. Her mermaid form features a violet tail. Specialized training allows her to briefly maintain human legs upon water contact, unlike most young mermaids.

Beginning sternly hostile toward the protagonists, Akeno's demeanor evolves into genuine compassion for Nagasumi and others, leading her to abandon her original mission. After Nagasumi helps her recognize her duty's flaws, her loyalty shifts and she joins his allies against antagonists. She harbors a paralyzing fear of ghosts, occasionally influencing her actions.

Her background reveals she is the younger sister of Seto group member Masa, a connection unknown to her until later events, adding complexity to her interactions. Her name derives from the Shiranui Sea and "Ake no Myōjō," referencing Venus as the morning star in eastern skies.

Initially acting under orders from the primary antagonist to dismantle Nagasumi and San's relationship, Akeno defects after witnessing Nagasumi's integrity. Her character arc concludes fighting alongside former foes to protect their bonds.