The guardians/mentors of powerful young senshi are... cats. These are not just ordinary cats; it turns out that they are special cats from the planet Mau, chosen to train and guard the senshi in their growing power. Artemis took on the task of training Minako to become Sailor V; Luna did the same with Usagi. They finally joined forces when the senshi became a team. Diana, their daughter from the future, is the guardian/companion of Chibiusa.
The cats do not actually join in the fighting. (Not usually, anyway.) Instead, they stay back and give advice to the senshi, and do research to figure out the enemies' weaknesses. They're very computer-savvy cats; they stay in touch with the senshi via headsets, Luna has a laptop, and she and Artemis operate the computers in the hideout under the Crown Game Center.
They all have crescent moon-shaped bald spots on their foreheads. When the moon is covered or damaged, they cannot speak or think clearly. (Well, Luna can't, anyway. It has never happened to the other cats, but I'll assume that it's the rule.)
The cats are exceedingly tolerant of their charges, and often inventive in finding new ways to train them. Artemis marked Minako's fondness for video games, and invented the "Sailor V" game to train her reflexes and reactions. Luna alternately bribed, cajoled, and lectured Usagi into becoming a good Sailor senshi. Artemis is the master of snide remarks, whereas Luna tends to be more serious. Diana just seems sweet, earnest, and playful... but Chibiusa isn't as much of a handful as Usagi or Minako.
Each cat is named after some aspect of the Moon. "Luna" is obvious; it's the Latin word for "moon." "Artemis" is the Greek goddess of virginity and the hunt; the moon was her symbol. "Diana" is the Roman name for the same goddess. Luna's coat is black and her eyes are blue; her human form is black-haired and petite, with odangos similar to Usagi's. Artemis is the white-furred, green-eyed cat; his human form is slim but broad-shouldered, and his long white hair falls behind him except for the strands in front, which are bound into a crescent at his chest. Diana is a cute little gray cat, who changes into a cute little gray-haired girl. Her eyes are a delicate pink.
It does not seem as if the cats have powers of their own, but they're obviously able to do something. When Diana offered to guard the space-time doors in Sailorpluto's place, Pluto didn't doubt the little cat's ability to handle it. Also, Luna and Artemis seem to be able to manipulate the arcade machines at the Crown Game Center to suit their purposes.
The cats have their share of emotional problems, too. Artemis has a close attachment to Minako, and sometimes it seems as if he's attracted to her. During Act 38, Artemis peeks at Mina while she's in the shower; Diana tells her mother that Artemis looks at Mina with the eyes of a lover. Luna shrugs it off, but then bites fiercely into Artemis' tail, saying that she won't let him peep into the bath anymore.
However, the tables are turned in Kaguya-hime no Koibito when Luna falls in love with a spaceman, Dr. Oozora Kakeru. (He saved her from being hit by a car.) Unfortunately, Kakeru's heart already belonged to another - the astronaut Nayotake Himeko. Luna discovered that Kakeru was being attacked by the legendary Princess Kaguya, a moon spirit; she called the matter to Sailormoon's attention, and they were able to save Kakeru. As a Christmas present, Usagi used her powers to briefly transform Luna into a human. Luna took Kakeru into space and kissed him; but she realized that he really belonged with Himeko, and being a cat really suits her better.
It's obvious that Artemis loves Luna; when he heard about her crush on Kakeru, he gets quiet and depressed, and he stared at her human form with longing in his eyes. However, he didn't make an issue of it, and when Luna settled back into her normal way of life, he quietly reestablished their close friendship.
(Trivia tidbit: At the beginning of the series, Usagi's mother called Luna "Mikatsuki hage-chan." "Mikatsuki" means "new moon" or "crescent moon," and "hage" means "bald." Hence, "crescent moon baldie"-chan was Luna's nickname. Obviously, Luna didn't like this very much...)