As a fan of Sailormoon (well we're assuming you are if you are here), you have probably noticed the theme of senshi names taken from mythology. Well Lethe and Mnemosyne are certaintly no exception!
Sailor Lethe is named after a river in Greek myhtology found in the underworld. The river was known as Lethe, or the River of Forgetfulness. Spirits of the dead would drink from this river before passing into the Elysian Fields (aka Heaven) so they will forget the sorrow of earthly life.
Naoko obviously read up on this because it is stated that Sailor Lethe is the guardian of the River of Oblivian.
Sailor Mnemosyne is named after the Goddess of Memory who was one of the original 13 Titans. Zeus falls in love with her and Mnemosyne gives birth to the nine Muses who are best known for their poetic talents. When a writer was seeking help, he would often pray to the Muses for help.
Sailor Mnemosyne is also the protector of a river, the River of Memory. Now doesn't this make the River Senshi an opposite pair? One is protecting memory while the other urges you to forget!