Michiru Kaiou was the civilian identity of Sailor Neptune, and appeared in the anime and manga series, as well as the musicals.
Michiru was mature, calm, and quiet, and it was hard to tell just what she was thinking. She had a distinct sense of humor, though, and enjoyed teasing Haruka. According to the Materials Collection, she had a kindness "as great as the sea." She was also a gifted artist and accomplished violinist. She often performed in concerts, either by herself or jointly, such as when she held a concert with the Three Lights.
She lived the life of the wealthy Tokyo elite and, in the Sailor Moon S season/Infinity arc, attended Mugen Gakuen, a school for genius students. At the time, however, she attended it to infiltrate its operations.
Michiru had no known relatives in either the manga or the anime series, and seemed to only live with Haruka. In the manga, however, she moved in with Haruka and Setsuna Meiou to raise baby Hotaru after she was reborn at the end of the Infinity arc, and the four of them formed an adopted family. In the English dub, she and Amara (Haruka) were cousins.
Michiru was romantically linked with Haruka Tenou. Although the anime was not graphic, there were hints given (especially during the Stars season) that Michiru and Haruka had a physical relationship. In the English dub, however, Michelle (Michiru) and Amara were changed to "cousins" in order to avoid controversy surrounding homosexuality.
In the musicals, Michiru was played by Kahoru Sakamoto, Chikage Tomita, Miyuki Fuji, Hiroko Tahara, Sara Shimada, Yuuka Asami, Tomoko Inami, and Takayo Ooyama.
- In 1998, Irwin released a line of Senshi of the Outer Solar System dolls well before Sailor Moon S aired in North America. On the boxes of the Sailor Neptune dolls, she was given the civilian name "Nerissa."
- When Megumi Ogata, Haruka's voice actress, asked Kunihiko Ikuhara how she was supposed to portray Haruka and Michiru's relationship, she was told to act as if they were a married couple.
- Michiru's skill with the violin extended to playing the instrument while simultaneously juggling a lemon.
- Michiru and Haruka seemed to own a helicopter and could pilot it themselves. In the manga, Michiru had her own helicopter named the Kaioumaru (海王丸; meaning "Ocean King").
- According to the Materials Collection, Michiru's character was influenced by the Takarazuka Revue.