Sailor Lethe and Sailor Mnemosyne are twin sisters who show up only in Act 48 of the manga. They hail from the planets Lethe and Mnemosyne—small, impoverished worlds ravaged by constant violence and chaos. When Galaxia arrived with her destruction, the sisters felt they had no choice but to join her, hoping she might finally bring them peace.
Mnemosyne takes her name from the Titaness of memory and mother of the muses, though the name also belongs to an underworld river said to grant remembrance and omniscience. Lethe, meanwhile, introduces herself as the ferryman of the desert river—a fitting role given what she does next.
Early on, Lethe traps Eternal Sailor Moon at the bottom of the River of Oblivion, kills Luna, Artemis, and Diana, and goes after Moon herself. But Mnemosyne stops her, arguing that Sailor Moon has suffered enough already. The sisters then hear Moon's reasoning: she believes Moon's Sailor Crystal—the Ginzuishou—is driving all the wars around them, and that destroying it would bring peace. Moon offers her own life to end the violence.
This act of self-sacrifice hits them hard. Lethe and Mnemosyne realize that Moon's death won't actually stop the fighting; the wars will keep happening either way. They're on the verge of letting her go when Sailor Chi and Sailor Phi show up and call them useless fools. The two newcomers strip Lethe and Mnemosyne of their Sailor Crystals, killing them.
Content compiled by AnimeList.moe from publicly available sources.
