Luna

Luna was a talking cat and the adviser to Usagi Tsukino. She was the one who first detected Usagi's powers, but did not realise she was the Moon Princess at the time. Luna and her partner Artemis were advisors to Queen Serenity in the Silver Millennium. When the kingdom was turned to ruins, Serenity used the power of her Silver Crystal to send the two cats to Earth's future in order to awaken the Sailor Senshi.

Manga
According to the manga, Luna and Artemis were originally from the planet Mau. Also in times of crisis she, along with Artemis and Diana, can briefly take on human form. In her human form, she had long black hair with two odango style buns on the side of her head and blue eyes.

Anime
Luna in the anime only took on human form once in the Sailor Moon S movie.

PGSM Series
In the live-action series, Luna was a talking and moving plush cat (not a real one). As a side effect of being bathed in the light of the Silver Crystal, she could become a human girl and transform into Sailor Luna to train and help the Senshi, but she turned back if she sneezed or was shocked. Her powers employed the use of candy, and she wielded a large folding fan at times. Fans viewed her as the replacement of Chibiusa for the sake of keeping Pretty Guardian Sailor Moon a largely one-series story. As a plush cat, Luna was dark blue(almost purple) instead of black, and her eyes were blue.