Sailor Lethe and Sailor Mnemosyne make their sole appearance 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 in hopes of eventually finding peace.
Sailor Mnemosyne is named after the Titaness of memory, who is also the mother of the muses, and another river in the underworld that brings remembrance and omniscience.
Sailor Lethe introduces herself as the ferryman of the desert river. She traps Eternal Sailor Moon at the bottom of the River of Oblivion, kills Luna, Artemis, and Diana, and 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. In response, Sailor Moon offers her life if it will end the violence. Moved by her self-sacrifice, Lethe and Mnemosyne realize that whether Sailor Moon lives or dies, the war will continue regardless. Just as they are about to release Sailor Moon, Sailor Chi and Sailor Phi appear. These two new arrivals call Lethe and Mnemosyne useless fools and take their Sailor Crystals, resulting in their deaths.
