Michiru is an
elegant girl who likes fashion dresses and make-up
things. She lives with Haruka, in S (I suppose) and, in
SailorStars, with her and Setsuna, raising Hotaru as
Michiru-mama. Her family isn't mentioned in the Anime.
Michiru likes playing violin, and does it really good; she even gives concerts. Now, even when neither Michiru nor Haruka accept they are lovers, they surely are, as we see in their scenes. They both appeared in S being hostile to the Inner Sailor Senshi till much later. She's maternal with Hotaru, and cares a lot for her. Other art occupation she has is painting, what she does good as well, and causes the admiration of Usagi.
Her name means Complete/Mature Sea Ruler and Neptune in Japanese means Sea King Star. Her power obviously relates to Sea. She has one of the talismans, the Deep Aqua Mirror, which predicts things and show the true form of things. In SailorStars she transforms into Super Sailor Neptune.