Sailor Mnemosyne (SMCU)

Sailor Mnemosyne of the Mnemosyne 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 Lethe, after their home planets' demise. After Galaxia was purified at the end of the Second Sailor Crusade, Mnemosyne later faces and rejoins Galaxia as a member of the Galaxia Troops.

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

Darkness Returns
Mnemosyne is the pacifist among the four mystery Sailor Senshi. She seemed to abhor violence, especially against other Sailor Senshi.