Artemis

Artemis is Sailor Venus' guardian. He is a cat, sent from the Moon Kingdom help Sailor Venus 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).