Sailor Neptune isn't a guardian Soldier to Princess Lady Serenity like the Inner Soldiers. Instead, she is a protector of the Solar System to keep invaders from threatening the Silver Millenium. She shares this destiny with Sailor Uranus and Sailor Pluto.

Being an Outer Soldier, Sailor Neptune bears one of the three talismans used in summoning Sailor Saturn, the soldier of Silence and Destruction. Her talsiman is a Mirror. She can see many different things in her mirror, such as visions of impending doom, a comet just entering the Solar System, a persons past life and lipstick shades that she can criticize.

When she was reborn on Earth, her mission was to defeat the Death Busters, a powerful enemy infiltrating Earth through the time space continuum. As Kaioh Michiru, she was a famous violinist, had a rich apartment on the Triangle Delta surrounding Infinity College, and was an exceptionally good swimmer.

In Sailor Moon Eternity and Sailor Moon Super, Neptune and Uranus didn't show any interest in working together with Princess Lady Serenity and her guardian Soldiers. She and Uranus were convinced they would just get in the way. Their goal was to kill Sailor Saturn and make sure the three talismans were never able to summon her. Unfortunately, Sailor Saturn turned out to be sleeping in Hotaru, Chibi-Usa's new best friend.