Sailor Lethe and Sailor Mnemosyne appear only in Act 48 of the manga. They are twin sisters and come from the planets Lethe and Mnemosyne, respectively. Their planets were small, poor, and plagued by violence and chaos. When Galaxia appeared and brought death and ruin, the two had no choice but to follow her in the hopes of someday finding peace.

Sailor Mnemosyne is named for the Titaness of memory, the mother of the muses, as well as another river in the underworld which brought remembrance and omniscience.

Sailor Lethe first 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. She is stopped by Sailor Mnemosyne, who says that Sailor Moon has been hurt enough.

Lethe says that Sailor Moon's Sailor Crystal, the Ginzuishou, is the cause for all of the wars they are experiencing, and that if it is destroyed, there will be peace. Sailor Moon offers her life if it will end the violence. Lethe and Mnemosyne are so moved by her self-sacrifice that they realize that whether or not Sailor Moon dies, the war will continue. Just as they are about to release the Senshi, Sailor Chi and Sailor Phi appear. These call Lethe and Mnemosyne useless fools and take their Sailor Crystals, which kills them.