Artemis is Mina's white cat. Before Sailor Venus joins the rest of the Sailor Scouts, Artemis is the cat that Luna talks to secretly, through the computer at the video arcade.
Artemis was Mina's cat, before the existance of the Sailor Scouts. He gave Mina her transformation pen, so that she could fight the world's evils as Sailor V. Believing that the forces of the Negaverse would return to Earth, Artemis was an important guide to help Luna to find the other Sailor Scouts.
Artemis is a cool cat with an attitude but eventually ( in the later seasons in Japan ) he and Luna have a child named Diana who comes back from the future ( just like Renie ). She is Renie's guardian cat, like Luna is to Serena.