She was introduced in the Sailor Moon S season of the anime and Infinity arc of the manga. Sailor Neptune fought alongside her partner and lover Sailor Uranus in defending the Solar System from outside threats. In both versions of the story, she was elegant, sophisticated, capable of sharp anger, and sometimes cold, whether she was in civilian or Senshi form. She was fully dedicated to her duty as a Senshi. Her powers were based on the sea or ocean (not just water in general like Sailor Mercury).
Sailor Neptune referred to herself as the "Soldier of Deep Waters" (深海の戦士) and the "Soldier of Embrace" (抱擁の戦士), under the guardianship of the "Sea Planet," referring to the literal meaning of the Japanese name for Neptune.