The Moon was a vampire and a member of the Vampire King's court.
Marceline fought The Moon centuries ago when she was on a mission to protect the remaining humans from vampires. The encounter was difficult thanks to The Moons regeneration abilities and her "hide and seek" style of defense. Marceline manages to defeat her and gain her healing powers.
The vampires are resurrected after Princess Bubblegum removes Marceline's vampire essence from her. After a short period, the Vampire King and his court manifest. While the other members of the vampire court debate what to do, The Moon calmly listens. When it appears that disagreements are breaking up the vampire court, The Moon parts ways with the group to go be off on her own. She heads towards a lake dropping moon pearls as she leaves.
The Moon is, at most times, an incredibly passive being, calm and quiet most of the time. She is filled with mystery and she takes advantage of this, to look as creepy as she can look.
However, when she is to hunt, her ways change drastically. She reveals a malicious interior which is also extremely dangerous, and never stops reflecting her mysterious aura. Incredibly twisted and scheming, she now talks in a deep, demonic voice and speaks in riddles and metaphors, causing a sense of confusion and fear in her pray. She's also more openly violent and angry.
The Moon has the appearance of a short, stout woman with long black hair and a serene expression. She wears a dark grey dress, a lighter grey poncho, and a golden pendant decorated with a crescent moon. Compared with the other vampires, she looks mostly human except for the bat ear growing out of the center of her forehead.