Other than Pluto, Sailor Neptune was the first Outer Senshi to get her powers. She's fairly strong, and she manages to be elegant and pretty, even when she's fighting! Her powers are based on the ocean.
Michiru's birthday is March 6th, so she's a Pisces. And of course, Pisces is ruled by the planet Neptune. Pisces are often very creative and intuitive people. Sound like any particular Sailor Senshi? Michiru often uses the sea to predict things that will occur. Another interesting tidbit is that Michiru was born on the Day of the Beauty Lovers. Isn't that just so like her?