Luna is a small black cat with a gold colored quarter-moon on her forehead. She knows most of the history of the Moon Kingdom and that she's supposed to find the Moon Princess. Of course when she meets up with Serena and enlists her help, she has no idea that Serena is that very princess, and neither does Serena or anyone else.
Serena originally meets Luna by accident. On her way to school - late as usual – she sees a bunch of primary school kids picking on something. She runs over and tells them to stop and Luna is revealed when they run away. As Serena picks up the black cat, she notices a Band-Aid on its forehead. At this point Luna opens her eyes and sees Serena. In her mind, she gets a strange feeling about the schoolgirl. As Serena peels the Band-Aid off, we get a glimpse of the moon crest there before Luna jumps up and runs away.
After school, Luna shows up in Serena's room; the cat has decided that this girl is important. Sailor Moon is then introduced as Serena's friend Molly is attacked by the Negaverse. Luna tells Serena to transform into Sailor Moon with some special words, but Serena thinks she's dreaming. The eventual outcome is that Sailor Moon appears finally and saves the day. Luna becomes the Sailor Scouts' mentor and is the character that knows the most about what the Sailor Scouts are supposed to do. Although enthusiastic in her quest for the Moon Princess, Luna sometimes gets a little impatient and frustrated with the teen-aged minds of her soldiers.