Luna is the Guardian cat of Sailor Moon, though she willingly offers advice and encouragement to the other Scouts.  (Isn't she nice?)  She was the servant of Queen Serenity in the Silver Millenium and was sent forward to the 20th century in suspended animation (kitty capsules!) to locate the Sailor Scouts and the lost Moon Princess.  She, although unknowingly, was communicating with Artemis through "Central Control."  Luna gave the Sailors their transformation pens and communicators.  She and Artemis will have a kitten, Diana, in 30th Century Crystal Tokyo.

Artemis is Sailor Venus' Guardian.  He and Luna were royal servants for Queen Serenity during the Silver Millenium.  The only memories he retained from that time when he awoke were those of his mission:  to find and awaken the Sailor Scouts, and to find the Moon Princess.  Artemis ran "Central Control", and gives sound advice (and insults!) when needed.  In 30th Century Crystal Tokyo, he and Luna have a kitten, Diana.