Sailor Lethe and Sailor Mnemosyne make their appearance solely in Act 48 of the manga. These twin sisters hail from the planets Lethe and Mnemosyne, respectively. Their homeworlds were small, impoverished, and marred by violence and chaos. When Galaxia arrived, bringing death and destruction, they had no choice but to follow her, hoping for eventual peace.
Sailor Lethe is named after a naiad from Greek mythology and the river in the underworld where souls go to forget their past lives before reincarnation. She introduces herself as the ferryman of the desert river and traps Eternal Sailor Moon at the bottom of the River of Oblivion. She kills Luna, Artemis, and Diana, and then attacks Sailor Moon. However, Sailor Mnemosyne intervenes, stating that Sailor Moon has already suffered enough.
Lethe claims that Sailor Moon's Sailor Crystal, the Ginzuishou, is the root cause of all the wars they are experiencing. She believes that destroying it will bring peace. Sailor Moon offers her life to end the violence. Moved by her self-sacrifice, Lethe and Mnemosyne realize that killing Sailor Moon won't stop the war. Just as they decide to release her, Sailor Chi and Sailor Phi appear. They call Lethe and Mnemosyne useless fools and take their Sailor Crystals, leading to their deaths.
Sailor Lethe possesses an attack called "Galactica Myosotis Alpestris," which she used against Usagi. Myosotis alpestris refers to an alpine species of Forget-me-not.
