- Gender: Female
- Age: Unknown
- Hair: Blue-Grey (human form) / Black (youma form)
- Eyes: Sea Green (human form) / Red (youma form)
- Nationality: Dark Kingdom
- From the anime Sailor Moon, property of Toei Animation Co.
- First Appearance: Chapter 6
One of the few, elite youma serving Queen Beryl directly rather than being under the command of any general in her army. Tethys has a wide variety of powers and skills, including the ability to create extremely large-scale illusions, animate water, teleport, direct high-pressure water attacks, scry across great distances, extend her limbs to great lengths and others.
Tethys has an infatuation with Jadeite, despite his indifference to her, which leads her to offering him her services in gathering energy. She also seems to be reasonably intelligent, if unlucky. Aside from this, not much is revealed about her in the Sailor Moon series.