Sailor Lethe and Sailor Mnemosyne only appear in the Bishoujo Senshi Sailor Moon manga. They do not appear in either the anime or the musical. Lethe and Mnemosyne are the guardians of the river in Shadow Galactica. Since the Shadow Galactica is mainly a desert, fans refer to these two as the Desert River Soldiers.

Lethe and Mnemosyne are often reffered to as villians in the Sailormoon manga, however, this is a misconception. Neither of the two soliders are evil or villian. Before the story of Sailormoon Stars, Galaxia had completely destroyed thier planets. They were than given a choice: die or live and work for Galaxia. Both girls choose to live and serve Galaxia, believing that she would be able to bring peace to the warring planets.

Sailor Lethe held a grudge against Sailor Moon, she believed that it was Sailor moon and the Ginzuishou that caused the Sailor Wars. When Sailor Moon, Kakyuu, the Starlights and Chibi Chibi reach the twin soldiers, Lethe is stopped by Mnemosyne in her attempt to kill Sailor Moon. After realizing that the Sailor Wars would not stop even with the death of Sailor Moon, they let the group go free. For this action, they are killed by Sailor Chi and Sailor Phi.