Sailor Mnemosyne of the asteroid "Mnemosyne" is a Sailor Senshi who joined Sailor Galaxia, along with her sister-senshi Sailor Lethe, after their home asteroids' demise. She only appears in the manga.
Unlike most of the Sailor Senshi under the control of Sailor Galaxia, Sailor Lethe and Sailor Mnemosyne were true Senshi, and felt they had no choice but to follow her. Before Galaxia's arrival, the planets of Lethe and Mnemosyne were always at war with one another, and the chaos resulting from the struggles was uncontrollable. Sailor Lethe and Sailor Mnemosyne joined Galaxia with the hope that she could bring about a better future for the universe, and end all war once and for all.
Mnemosyne was the passive one. She seemed to abhor violence, especially against other Sailor Senshi, and this was what caused her to stop Lethe from attacking Sailor Moon. When Mnemosyne asked Lethe to leave Sailor Moon's fate up to Galaxia herself, Sailor Moon managed to remember who she was and resurfaced with Sailor Chibi Chibi.
Though Mnemosyne seemed to share Lethe's strong belief that Sailor Moon and the Silver Crystal were the cause of all of their troubles, this belief was shattered when Sailor Moon offered to Lethe and Mnemosyne the simplest solution to their problems: to kill her. Sailor Moon's offer of self-sacrifice changed both Lethe and Mnemosyne's minds about Sailor Moon's true nature, and they realized that Sailor Moon came to Shadow Galactica to end war, not to cause it.
Just as Lethe and Mnemosyne allowed Sailor Moon's party to pass through, Sailor Phi and Sailor Chi and killed them, taking the Sailor Crystals of the Sailor Starlights as well as Lethe's and Mnemosyne's. It is a possibility that when all the Sailor Senshi's Star Seeds were freed from the Galaxy Cauldron and thus resurrecting the Senshi, Mnemosyne was also revived
- In Greek mythology, Mnemosyne is the name of the goddess, Mnemosyne, a Titan associated with memory and the mother of the Muses, And the Underworld river/pool named Mnemosyne for the goddess Mnemosyne.