Luna

Luna is a cat and Usagi/Sailor Moon's partner. She is a black cat with a crescent moon on her forehead. Many people mistake for a bald spot. She used to guard Princess and Queen Serenity in the Silver Millennium, along with Artemis.

How she met Usagi in the anime is when Usagi finds that three little kids were hurting Luna, but Usagi forced them to go away. But in the manga, she accidentally steps on Luna.