Mnemosyne is one-half of the conjoined twin planets. Lethe and Mnemosyne and the homeworld of Sailor Mnemosyne. As it is a conjoined twin asteroid, it has Sailor Mnemosyne as its Sailor Senshi. Like its twin Lethe, it was small and poor and ongoing war and chaos always ensued. Both Mnemosyne and Lethe were also close to dying when Sailor Galaxia came, and the twin Senshi sided with her willingly, believing that she would end the bloodshed.
- Mnemosyne is named after the Titaness "Mnemosyne", the godess of memory.