To gather energy from humans
Sailor Moon's Moon Tiara Action
Thetis volunteered herself to work with Jadeite in the twelfth episode. She was also one of the few youma to be shown with any real personality and depth, with her adoration for Jadeite and determination to prove herself to him.
Thetis who was monitoring the progress of Jadeite suggested that he may use her help. Initially reluctant, Jadeite was impressed with her idea of inviting human couples on a romantic cruise between Tokyo and Yokohama to steal their love energy. Although the ship looked impressive, it was nothing more than a figment of the imagination of the youma and Jadeite and in reality, it was a wrecked vessel built out of Thetis' seawater power and used only for her scheme which was dubbed "Operation Romantic Cruise”. Jadeite disguised himself as the ship's captain, while Thetis was disguised as his first mate.
Constant advertisement and promotions of the romantic cruise proved too much for Usagi Tsukino as she gave into the temptation of taking part in the cruise. To do this she was determined in entering a competition to take part in the cruise but was upset at Rei who unfairly won the competition and invited Ami instead. A jealous Usagi sneaks on board the ship as a photographer, and brings Luna with her. Luna had bad feelings of the ships captain, and this was confirmed when they witness the captain and his first mate (Thetis) mysteriously disappear in the captain's cabin after inspecting a strange looking machine which was used to collect the love energy of human couples with dates. The strange looking machine is activated once romantic energy has reached its maximum peak. Consequently, Thetis and Jadeite asked all of the passengers to go to the reception hall for a spectacular extravaganza. When everyone was inside, Thetis and Jadeite lose their disguises and Thetis with the aid of the strange looking machine began collecting everyone's energy using a disco ball high above the room. The device only gathered love energy to those with dates, however, so Ami and Rei were unharmed. Thetis sent her water monsters (disguised as crew members) to destroy Ami and Rei, but luckily, Sailor Moon arrived just in time. Thetis lunged at Sailor Moon and pushed her out of a window, then began attacked her by manipulating the sea water into columns which was hurled at Sailor Moon. As Jadeite joined in to watch the fight, Ami and Rei transformed into Sailor Mercury and Sailor Mars. Sailor Mars destroyed the crew members with her Fire Soul attack and then with Sailor Mercury, went to help Sailor Moon. Sailor Mercury used her Sabão Spray attack to confuse Thetis and Sailor Mars used her Fire Soul attack to incapacitate Thetis. The Fire Soul attack gave Sailor Moon the opportunity to finally defeat the youma with her Moon Tiara Action attack.
Unlike most youma, Thetis did not simply turn into dust after being struck by the Tiara. Instead her life energy burst out of her in rays of light. She struggled to remain standing but eventually collapsed to her knees and turned into water before she died. Once she was destroyed, the ship returned to its normal form, and all of the energy-drained passengers were revived.
Upon reporting back to Queen Beryl, Jadeite was grilled for having another plan foiled and costing her the life of Thetis.
- Thetis is the first youma to be one of the most powerful servants of Queen Beryl, later after her death were the Seven Great Youma.
- Thetis is named after a sea nymph from Greek mythology; she is also often mistakenly called Tethys, after a sea goddess.