She is the Guardian of Sailor Moon (Serena). The two of them met after Serena chased away some bullies who were scaring the cat to death. The cat had a bandage on its forehead and Serena pulled it away to reveal a crescent moon. Later, the cat introduced itself as Luna, and told her that she had been searching for Sererna for a very long time, that she was Sailor Moon. She gave Serena a special transformation locket to help her transform into the courageous Sailor Scout and has helped Serena ever since along the path of her destiny. Luna had a special job in the days of the Moon Kingdom, just like the other Scouts. She was an advisor to Queen Serenity along with Artemis.
Artemis is the guardian of Sailor Venus (Mina). He is a little more laid-back than Luna and he is always goofing off in some way that teases her. Nevertheless, he too is also a good guardian of the Sailor Scouts and helps them out with advice whenever possible. And once in a while, he comes through to save the day when the Scouts need help, risking himself in the face of danger.
Diana is Luna and Artemis's daughter from the future. A cute gray kitten, she is very spunky, energetic and inquisitive. She acts as Rini's guardian and loves riding around on her head. Diana also can't help calling Darien "your majesty" out of respect that he is king of Crystal Tokyo.