Thetis

テティス

Thetis is a female youma directly controlled by Queen Beryl in episode 012. Her mission is to assist Jadeite in gathering energy from humans to revive Queen Metallia. She is a powerful youma capable of controlling seawater. She seemed to harbor romantic feelings for Jadeite, appearing before him at the start of the episode with a plan to help him steal energy. Her plan was titled "Operation Romantic Cruise," involving the setup of a romantic cruise to steal the passengers' love energy. Although the ship appears beautiful, this is merely an illusion; it is actually a wrecked vessel. Jadeite disguised himself as the ship's captain, while Thetis disguised herself as a crew member. Rei manages to win tickets to board the ship and invites Ami. Usagi sneaks aboard using her disguise pen, bringing Luna with her.
Once the love energy reached a peak, Thetis and Jadeite asked all 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 lavendar 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.