Michiru Kaio attends Mugen Academy and is a celebrated violinist, radiating elegance and sophistication. Her civilian identity presents her as a wealthy teenage girl with wavy teal hair, deep blue eyes, and a refined fashion sense, frequently seen in white or green clothing outside her school uniform. She pilots a personal helicopter named "Kaiohmaru." Awakening as Sailor Neptune independently, without guidance from the moon cats or other Guardians, she serves as the Guardian of the Deep Seas. Her powers, tied to Neptune, the Roman sea god, encompass generating and manipulating seawater, creating tsunamis, and exhibiting precognition. Her signature attacks include "Deep Submerge" (summoning energy blasts or aquatic spheres), "Submarine Reflection" (using her Deep Aqua Mirror talisman to reveal truths or attack), and "Submarine Violon Tide" (channeling power through her violin, the Marine Cathédrale). The Deep Aqua Mirror also enables astral projection and barrier creation. In Sailor Moon Crystal Season III, Michiru first appears when Mamoru Chiba accidentally knocks her mirror to the ground. Noting his noble demeanor, she suggests he may have been a prince in a past life, demonstrating her mirror's ability to reveal past lives. Later, Usagi Tsukino discovers her playing violin at Mugen Academy, where Michiru issues a cryptic warning against interference. Alongside Haruka Tenou (Sailor Uranus), she investigates the academy's sinister activities, offering Chibiusa a ride home and revealing her mirror as a talisman. Sensing evil during a violin concert, she abruptly leaves the stage. Formally debuting as Sailor Neptune, she saves Sailor Chibi Moon from Mimete using "Deep Submerge," declaring herself a Guardian of the Outer Solar System. She later fights alongside Sailor Uranus and Sailor Pluto, bowing to Princess Serenity (Sailor Moon). Hypnotized by Cyprine, she battles the Inner Guardians but is freed by Sailor Moon. Her Deep Aqua Mirror resonates with the Holy Grail, enabling Sailor Moon's transformation into Super Sailor Moon and awakening Sailor Saturn. She supports Sailor Uranus' plan to eliminate Hotaru Tomoe to prevent Sailor Saturn's destructive awakening but fails to stop Mistress 9 from seizing Chibiusa's Silver Crystal. In the climax against Pharaoh 90, she envelops Mugen Academy in a protective barrier but is overwhelmed when it shatters. Her character arc includes a romantic relationship with Haruka Tenou, explicitly confirmed across all media. They share a deep bond, willing to sacrifice themselves for their mission or each other. After Mugen Academy's destruction, they join Setsuna Meiou (Sailor Pluto) to raise the reborn Hotaru, forming an adopted family. Sailor Neptune undergoes multiple transformations: her base form features a teal sailor collar, dark teal skirt, and wrist-length gloves. As Super Sailor Neptune, activated by the Neptune Crystal gifted by Sailor Saturn, her uniform gains a striped collar, elongated back bow, and elbow-length gloves. Her Eternal form includes a two-layered skirt, knee-high boots, and star-shaped accessories. Beyond Season III, she briefly falls under Sailor Galaxia's control in the Stars arc, betraying fellow Guardians to steal their Star Seeds. She and Haruka later reveal this as a ploy to defeat Galaxia, accepting death while affirming their commitment to each other. They are ultimately revived.