Sailor Lethe (SMCU)

Sailor Lethe of the Lethe system is a Sailor Senshi who is one of the mysterious Sailor Senshi who have joined Sailor Galaxia's Shadow Galactica Legion during the inter-war period of the Sailor Crusades, along with her sister-Senshi Sailor Mnemosyne, after their home planets' demise. After Galaxia was purified at the end of the Second Sailor Crusade, Lethe later faces and rejoins Galaxia as a member of the Galaxia Troops.

Prior to the Cinematic Universe
As the Soldier of Truth and twin of Sailor Mnemosyne, Sailor Lethe and her sister joined the Shadow Galactica Legion after her planet was destroyed around the 5th century AD in the inter-Crusade period.