Gaurdian Artemis

Artemis is Minako's Cat Gaurdian. He was introduced into the series the same episode as Sailor Venus. However, he is the Master Computer which gives Luna advice throughout the series. This isn't revealed until after he arrives with Sailor V.

Artemis is actually in love with Luna. This isn't really stated until their child (or kitten, whichever you prefer), Diana, arrives from the future. Even though, you can pretty much tell Artemis and Luna like each other. Also, Artemis is actually human. This is revealed in the manga during the Starlight episodes.