Luna

Luna is Sailor Moon's guardian. She is a cat, sent from the Moon Kingdom to watch over her and guide her. Initially, her mission is to help Sailor Moon find the long lost Princess Serenity, until it is revealed that Sailor Moon is in fact the princess.
Mythology: Selene (Greek for "moon") was the Greeks' first goddess of the moon. The Roman version of Selene is Luna (Latin for moon) and the Roman version of Artemis is Diana... thus the names for the three cats (Luna, Artemis, and Diana).