Thetis is a female youma under the direct control of Queen Beryl in episode 012. Her mission is to help Jadeite gather energy from human beings in order to revive Queen Metallia. She is a very powerful youma who can control seawater. She seemed to have romantic feelings towards Jadeite, and appeared before him at the beginning of the episode with an idea to help him steal energy. Her idea was called "Operation Romantic Cruise," where they would set up a romantic cruise and steal the passengers' energy of love. Although the ship looks beautiful, this is only an illusion; it is really a wrecked vessel. Jadeite disguised himself as the ship's captain, while Thetis was disguised as a crew member. Rei manages to win tickets to board the ship, and she invites Ami. Usagi sneaks onto the ship using her disguise pen, and brings Luna with her.
Once the love energy reached a peak, Thetis and Jadeite asked all of the passengers to go to the reception hall for a special romantic show. When everyone was inside, Thetis transformed into her youma form and began collecting everyone's energy using a disco ball high above the room. The device only gathered love energy, however, so Ami and Rei were unharmed. Thetis sent various monsters (disguised as crew members) to destroy Ami and Rei, but luckily, Sailor Moon arrived just in time. Thetis attacked Sailor Moon and pushed her out of a window, then began attacking her with swirling streams of sea water. Sailor Mars and Sailor Mercury quickly transformed and destroyed the monsters attacking them, then went to help Sailor Moon. They used their "Fire Soul" and "Sabão Spray" attacks to incapacitate Thetis, and Sailor Moon eventually used her "Moon Tiara Action" attack to completely destroy the youma. Once she was destroyed, the ship returned to its normal form, and all of the energy-drained passengers were revived.
In her human form, Thetis has light skin, blue hair, and dark green eyes. She wears a short blue dress with light blue gloves and white shoes. In her youma form, she has chalk white skin, dark blue (almost black) hair, and glowing red eyes. She does not appear to have a mouth, but her jaw moves when she speaks. She wears a blue, sleeveless, skin-tight suit with lavender gloves that reach past her elbows. She also has an elaborate gold headdress.
- Thetis's name is derived from the name of the Greek goddess Thetis who was one of the most well-known Nereids. The Nereids were sea nymphs who were helpful towards sailors.