Luna is a pretty black cat with red eyes. Along with Artemis, her job is to help train the Sailor Senshi. She acts kind of like Usagi's mother, always telling her what she should be doing (homework...) Luna gives lectures a lot, and hates wasting time on non-important activities (like shopping and playing video games). By emitting a moonbeam on her forehead, she can induce past memories. Also, by doing somersaults in the air, she produced the first 5 Senshi's magical items. In the Sailor Moon S movie, she briefly turned into a human, Kaguya Hime, a moon goddess.
Artemis' job is pretty much the same as Luna's, to teach and train the Sailor Senshi. He first appeared with Sailor V. He's not as worried about homework and the likes.
Diana is Luna and Artemis' purplish-grey kitten, with big red eyes and big ears. She came from the future in Sailor Moon Super S and acts as Chibi Usa's teacher. She likes to sit on Chibi Usa's head.