Thetis

In her human guise she keeps light skin, dark teal hair and teal eyes, dressing in a short sleeveless outfit with white gloves and a necklace echoing the crown she wears as a monster. Once transformed, she trades that look for a dark blue sleeveless bodysuit and gloves in a paler blue tipped with three long spikes along each side. Her hair holds the same style but grows far longer and turns deep blue, her skin shifts to a light blue, and every feature but her eyes fades away, those eyes now glowing red. A broad moon-shaped ornament sits atop her head, with a small crescent marked beneath her right eye.

Thetis stands out as one of the rare monsters written with real depth. She adores Jadeite and is driven to earn his regard, which pushes her to step forward on her own. She puts herself forward to serve alongside him in the twelfth episode, eager to prove her worth where blunter monsters would simply follow orders.

Watching Jadeite's efforts stall, Thetis proposes she lend a hand. He is cool to the idea at first but warms once she pitches a plan to lure human couples aboard a glamorous voyage running between Tokyo and Yokohama, all to skim their love energy in a scheme called Operation Romantic Cruise. The vessel looks luxurious yet exists only as a shared illusion; in truth it is a battered wreck she raises and steers with her power over seawater. Jadeite poses as the captain while she plays his first mate.

Relentless promotion of the cruise wears down Usagi Tsukino, who is desperate to join. She fails to win a contest that Rei takes instead, and Rei brings Ami along, leaving a jealous Usagi to slip aboard disguised as a photographer with Luna in tow. Luna's unease about the captain proves right when the pair vanish inside his cabin after tending a strange device meant to gather the energy of dating couples. The machine triggers once romance in the hall reaches its peak, so Thetis and Jadeite herd every passenger into the reception room for a promised spectacle. There they drop their disguises and, using a high disco ball, Thetis begins draining the crowd, though only those with dates are affected, sparing Ami and Rei. She sends water-formed servants dressed as crew to kill the two, but Sailor Moon reaches them in time.

Thetis throws Sailor Moon through a window and hammers at her with columns of raised seawater as Jadeite looks on. Ami and Rei transform, and Sailor Mars burns down the golem crew with Fire Soul before joining Sailor Mercury to help. Bubble Spray blinds Thetis, a second Fire Soul staggers her, and Sailor Moon finishes her with Moon Tiara Action. Rather than scatter as dust, her life energy erupts in beams of light; she wavers, sinks to her knees, and melts into water as she dies, making her the first monster to dissolve that way. With her gone, the ship reverts to its ruined state and the drained passengers wake. Jadeite moves to finish the Guardians, but Beryl yanks him back and berates him for letting her personal monster fall.