TV-Series
Description
Mermaid Thetis is a servant of Poseidon, the god of the seas, and serves as an assistant to his Mariner Generals. She is a Marina of lower rank than the Generals, possibly corresponding to the level of a Silver Saint in Athena's army. Her true form is that of a mermaid, though she can assume a human appearance. Thetis has blonde hair and blue eyes, stands 165 centimeters tall, and weighs 52 kilograms. She is fifteen years old, of Danish nationality, and was born on November 21.

Thetis's background is defined by a personal debt of life. When Julian Solo was a young boy, he found a beautiful fish stranded on a beach and tossed it back into the sea. That fish was Thetis. Years later, during Julian's sixteenth birthday party, Thetis lured him to Cape Sounion to awaken his identity as the reincarnation of Poseidon. Her actions are driven by profound gratitude and a deep sense of loyalty toward Julian, which eventually grows into genuine personal affection that goes beyond her duty to the god he hosts. Her loyalty is so absolute that she is willing to sacrifice everything for his safety.

In terms of personality, Thetis is calm, dutiful, and cautious. She frequently warns about potential threats, advising the Mariner Generals not to underestimate Athena and her Saints. She is also manipulative when necessary; she uses her beauty and charm to achieve her goals, such as when she draws Julian to the sea. Despite her loyalty, she is not cruel or hateful, and she demonstrates a capacity for genuine care, especially in her final moments.

Thetis's role in the story is primarily as a messenger, scout, and mediator for Poseidon's forces. She does not guard one of the seven pillars, which distinguishes her from the Mariner Generals. Early in the Poseidon arc, she invades the Kido mansion alongside other Marinas in an attempt to kidnap Saori Kido. During this encounter, she reveals that Poseidon is responsible for the global catastrophes. She later delivers Poseidon's declaration of war to the old master Dohko at the Five Old Peaks of Rozan. Throughout the conflict, she also explains to the Bronze Saints how they can save Athena by destroying the seven pillars.

Her key relationships include her bond with Julian Solo, which is the emotional core of her character. She feels a deep personal gratitude and affection for him, distinct from her reverence for Poseidon. She also interacts with the Mariner Generals, particularly Kanon, the Dragon of the Sea, who reprimands her when she is followed by intruders into the Undersea Temple. Her primary adversary among the Saints is Shaina of Ophiuchus, who defeats her in battle. Despite being defeated, Thetis warns Shaina that Julian Solo has become a god and that the Saints will never defeat him. She also briefly clashes with Kiki of the Appendix and, in the anime, with Seiya of Pegasus and Shun of Andromeda.

Thetis undergoes a clear arc from a devoted servant to a self-sacrificing protector. After Athena removes Poseidon's soul from Julian and the Underwater Sanctuary begins to collapse, Thetis reappears despite her earlier defeat. She rescues the unconscious Julian, honoring her promise to save the one who once saved her. As she dies, she reverts to her fish form. Julian, now free from Poseidon's influence, finds the fish on the shore, feels a deep sense of familiarity, and returns it to the ocean.

In terms of abilities, Thetis is not primarily a combatant. Her main function is as a messenger, and her fighting skills are limited compared to the Mariner Generals. Her official stats indicate average physical abilities in punching, kicking, technique with weapons, psychic attacks, and strength, but her overall effectiveness relies heavily on her cosmo energy. She is known for her agility and quick movements, which she uses to confuse opponents. In the anime, she displays a unique technique called Coral Trap, or Death Trap Coral, in which she fires beams of light shaped like coral that hypnotize enemies through their beauty and the sound of her siren song. This ability is exclusive to the anime adaptation and does not appear in the original manga.