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