Edomae Luna is a mermaid distinguished by pink hair, pink eyes, and a yellow tail fin, often clad in her Isono Junior High School uniform. Her legs automatically revert to a mermaid tail upon water contact, requiring thorough drying to restore her human form. She consistently refers to herself using plural pronouns ("we," "us"), even internally, though the reason is unknown. A trademark laugh frequently heralds her arrival.

Her personality exhibits extreme contrasts, oscillating between a manufactured, cutesy idol persona and a domineering, violent, self-centered nature. She displays arrogance, entitlement, psychopathic tendencies, and a need for control, frequently treating others—including the Michishio family housing her—as subordinates or "slaves." This behavior masks a profound craving for affection and validation, rooted in a dysfunctional family: her father exhibits socially inept, obsessive behaviors like stalking and cross-dressing, while her mother abandoned her at age five. Luna pursued pop stardom primarily to attract her mother's attention through television, leveraging her natural mermaid singing voice to manipulate audiences for fame. She harbors a superiority complex, especially towards childhood friend and rival Sun Seto, fueled by unresolved inferiority stemming from losing a youth singing contest to Sun. Despite their rivalry, Luna maintains a more genuine, less performative connection with Sun, uniquely referring to her by name.

Luna's initial entanglement with human Nagasumi Michishio stemmed from her rivalry with Sun. Misinterpreting Sun's affection for Nagasumi as boasting, Luna resolved to "steal" him, exploiting his knowledge of her mermaid form to force him into servitude and move into the Michishio home. While early actions like learning to cook for him were framed as competitive moves against Sun, she eventually developed authentic romantic feelings for Nagasumi. This culminated in a marriage plan halted by Sun's intervention. Despite the canceled wedding, Luna continued residing with Nagasumi's family. Over time, her violent outbursts and true personality surfaced more often publicly, possibly indicating increased comfort or less effort maintaining her idol facade. The manga depicts her expressing a desire to stay with Nagasumi and Sun indefinitely, suggesting a complex emotional bond.

As a mermaid, Luna shares the common ailurophobia (fear of cats). Her primary ability is her voice, wielded as a pop idol. Specific songs can manipulate emotions and behavior—her "war song" incites violent conflict. She also emits an ultrasonic scream causing physical damage and stunning opponents. Luna commands a personal army of boys from Isono Junior High, deploying them like a private militia; this group perpetually clashes with Sun’s bodyguards.