As the Guardian of Truth and twin of Sailor Mnemosyne, Sailor Lethe and her sister joined Shadow Galactica after her planet was destroyed. When Sailor Moon arrived to confront Galaxia, she was confronted by an unbearable heat and a desert, and then came upon a river, where a mysterious boatmen offered her a ride. However, she soon awakes at the bottom of the river, her memories gone, and Lethe attacks her.
Unlike most of the Sailor Guardians under the control of Sailor Galaxia, Sailor Lethe and Sailor Mnemosyne were true Sailor Guardians, and they felt they had no choice but to follow her. Before Galaxia's arrival, the planets of Lethe and Mnemosyne were always at war with one another, and the chaos resulting from the struggles was uncontrollable. Sailor Lethe and Sailor Mnemosyne joined Galaxia hoping that she could bring about a better future for the universe, and end all war once and for all.
Of the two Desert River Sailor Guardians, Lethe was the more aggressive one. Not only did she attack Sailor Moon while she was unable to fight back, but she also killed Luna, Artemis, and Diana in order to make her point about how futile Sailor Moon's life was. Lethe's vengeance was thwarted by her sister, Sailor Mnemosyne, who inadvertently allowed Sailor Moon to remember who she is and resurface with Sailor Chibi Chibi. Despite her betrayal, Lethe could not bring herself to attack her sister, as they both shared the same beliefs and convictions; instead, she reassured Mnemosyne that Sailor Galaxia would make everything all right for them.
Lethe held a strong belief that Sailor Moon and the Silver Crystal were the cause of all of their troubles, and said as much to Sailor Moon and Princess Kakyuu when they confronted each other. Lethe's belief in Galaxia was then shattered when Sailor Moon offered to Lethe and Mnemosyne the simplest solution to their problems: to kill her. Sailor Moon's offer of self-sacrifice changed both Lethe and Mnemosyne's minds about Sailor Moon's true nature, and they realized that Sailor Moon came to Shadow Galactica to end war, not to cause it.
Just as Lethe and Mnemosyne allowed Sailor Moon's party to pass through, Sailor Chi and Sailor Phi appeared and killed them, taking the Sailor Crystals of the Sailor Starlights as well as Lethe's and Mnemosyne's. It is a possibility that when all the Sailor Guardians' Star Seeds were freed from the Galaxy Cauldron and thus resurrecting the Senshi, Lethe too was revived.
Her attack is Galactica Myosotis Alpestris, an energy blast that was thrown at enemies. The name "Myosotis Alpestris" is also the scientific name for the flower known as "Forget-Me-Not".
- In Greek mythology, Lethe was one of the five rivers of Hades.
- "Lethe" also means "oblivion" in Greek.