Diana, like ChibiUsa, is a guardian cat sent from the future (Crystal Tokyo). She is eventually revealed to be the future daughter of Luna and Artemis.
Mythology: Selene (Greek for “moon”) was the Greeks’ first moon goddess. Luna (Latin for moon) is the Roman equivalent of Selene, and Diana is the Roman equivalent of Artemis… therefore the names of the three cats (Luna, Artemis, and Diana).