Sailor Mau is the Sailor Senshi of the planet "Mau", the homeworld of Luna, Artemis, Diana, & Tin Nyanko. Though Sailor Mau-herself never appeared in the manga, it's implied that Sailor Tin Nyanko killed her in order to get her star seed and gain Sailor Galaxia's favor. Luna and Artemis holds a personal grudge against her for killing Sailor Mau, after having learned of the fact.