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Unlike most of the Sailor Senshi under the control of Sailor Galaxia, Sailor Lethe and Sailor Mnemosyne were true Senshi, and they 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.
Of the two Desert River Senshi, 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 (a difficult feat, given that she had swallowed water from the Desert River of Oblivion) and resurfaced with Sailor Chibi Chibi. Mnemosyne also helped to revive the memories of Princess Kakyuu with water from the Desert River of Memory. Afterward she hid behind Lethe, allowing her to confront Sailor Moon for the two of them.
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 appeared and killed them, taking the Sailor Crystals of the Sailor Starlights as well as Lethe's and Mnemosyne's.