Luna is the cat who first tells Usagi that she is Sailor Moon. It started as Usagi rescuing her from some boys when she was on her way to school. Later on that day Luna came into Usagi's room and started talking to her, which of course freaked Usagi out. Luna finally convinced her that she was Sailor Moon after doing her back flip and producing the Moon Locket, having her transform, and having her rescue Naru.
Luna is like the mother of the scouts. She tells them what they should and what they shouldn't do, advice that is followed during missions, but not in real life.
She loves to tease Artemis about being a clutz, but it's hinted at that she really likes him as more than a friend, and this is proven when Diana shows up from the future.
But before Diana shows up, in the S movie, Luna falls in love with a human. Since she is a cat, she wishes that she could become Princess Kaguya of the Moon, the person that this man has wanted to meet his whole life. Super Sailor Moon uses the Silver Crystal and lets Luna become Princess Kaguya for a day so she takes him with her to the Moon.
Artemis is the cat who helped Minako when she was in London. I don't really know how exactly he informed her that she was Sailor V and Sailor Venus.
He is somewhat of a clutz, and Luna loves to tease him about it.
Luna and Artemis's daughter of the future, she is Chibi-Usa's Guardian Cat. When she first showed up, she got Artemis in trouble with Luna by calling him daddy, she hadn't been told yet that Diana was her daughter.