Mylène, better known as Sailor Neptune, is the civilian identity of a teenage girl named Michiru Kaioh, who appears in the Sailor Moon SuperS season of the anime. She is a member of the Outer Sailor Guardians, a group of warriors tasked with protecting the solar system from external threats. In the SuperS season, her role is notably reduced compared to her prominent introduction in the previous season, Sailor Moon S. She appears primarily in the SuperS television special and the theatrical film Sailor Moon SuperS: The Black Dream Hole, rather than throughout the entire series.

Her background establishes her as a highly talented and accomplished individual. She is a world-renowned violinist and a gifted painter, with a deep passion for the arts. She is also an exceptional swimmer. She comes from a wealthy and sophisticated background, attending the prestigious Mugen Academy alongside her partner, Haruka Tenoh. In SuperS, her civilian life is shown to have settled into a domestic routine. She, Haruka, and Setsuna Meioh live together in a family-like arrangement, with Michiru focusing on her career while also caring for the younger Hotaru Tomoe. Their home serves as a quiet sanctuary away from the main action of the season.

Personality-wise, Mylène is characterized by a serene and elegant demeanor. She is consistently polite, calm, and graceful, projecting an aura of refinement and maturity that sets her apart from the younger Inner Guardians. This exterior, however, masks a layer of fierce determination and a willingness to make difficult, morally ambiguous choices for the sake of her duty. While she is kind and compassionate, she can be sharply critical and even cold when she believes others are not taking their responsibilities seriously. She possesses a strong sense of conviction and is not afraid to act on her beliefs, even if it means clashing with allies.

Her primary motivation throughout Sailor Moon SuperS remains the same as in her earlier appearances: the unwavering commitment to her mission as a Sailor Guardian. She believes in protecting the world and the solar system at any cost, and she views her duty as paramount. This sense of responsibility is deeply intertwined with her personal relationships, particularly her bond with Haruka Tenoh, Sailor Uranus. Their relationship is the cornerstone of her character; they are inseparable partners, lovers, and a unified fighting force. In SuperS, their dynamic is portrayed as a settled, deeply trusting partnership. They continue to operate as a pair, often acting independently from the main group, relying on their own judgment and shared history. A key moment in the SuperS special, the Dream Hole film, reinforces the depth of her loyalty: when a villain forces her to choose between saving Haruka and saving the world, she attacks without hesitation, declaring that a world without Haruka is not worth protecting. This illustrates that her love for Haruka is as central to her identity as her sense of duty.

Development in SuperS is subtle. Having already undergone significant growth and internal conflict in the S season, she enters SuperS with a more settled and confident sense of purpose. She has fully accepted her role and her partnership, and the season shows her in a more domestic and stable phase of life. She acts as a guardian from the shadows, remaining observant and ready to intervene when necessary, but largely allowing the Inner Guardians, now powered up by Pegasus, to take the lead. Her primary growth is in the deepening of her familial bonds with Haruka, Setsuna, and Hotaru, showing a softer, more nurturing side.

As a Sailor Guardian, Sailor Neptune commands the power of the oceans. Her abilities are vast and tied to the element of the sea, which she can summon, manipulate, and wield offensively. She also possesses strong precognitive abilities, often sensing danger and gaining insights into the future through her intuition and the state of the sea. Her signature attack is Deep Submerge, a powerful torrent of ocean energy. Her most important item is the Deep Aqua Mirror, one of three mystical Talismans. The mirror can reveal truths and foresee events, making her a valuable strategist. In the SuperS movie, she is also seen using her upgraded form, Super Sailor Neptune, which amplifies her existing powers. In the season, her role is more supportive and observational, providing crucial intelligence and powerful backup when the main team faces overwhelming threats. She embodies the qualities of a guardian who watches, waits, and strikes with decisive force when the moment is right.