Mermaid Thetis

Mermaid Thetis (人魚姫のテティス, Māmeido no Tetisu) is a servant of Poseidon, the god of the Seas, and assistant to his Marine Generals.

Thetis, the Mermaid of Poseidon
Thetis, without her helmet, in a panel from the manga in vol.14. Drawing by the author Masami KurumadaKurumada introduced Thetis in the chapter 46 of his manga, contained in vol.14, and in the anime adaptation in episode 100.

Saori Kido attended billionaire heir Julian Solo's birthday party at Cape Sounion, Greece. There she was surprised by Julian's marriage proposal, which she refused. Minutes after the party, Julian was visited by a mysterious young woman who told him that he had to fulfill his destiny, as he had been chosen by Poseidon, Emperor of the Seas, as the body he needed to reincarnate. The young woman took Julian to the sea and he fainted minutes later underwater.

This woman was Mermaid Thetis, faithful servant of Poseidon, who, like the Seven Marina Generals, had sensed Poseidon's awakening after two thousand years of sleep.

Later, Saori Kido Athena, sensing Poseidon's return, met with Julian Solo, who by then had received Poseidon's soul. Poseidon wanted to cleanse Earth of humans who had done so much harm, with heavy rains and flooding. Athena tried to convince him that not all humans were evil and that there was hope for them, but Poseidon wasn't convinced. She then offered herself as sacrifice, to delay the destruction of Earth, and Poseidon locked her away in the Main Breadwinner. If she wanted to save Earth, then she would have to endure the waters and stop them somehow, by herself or with the help of her fabled Saints.

Thetis was sent to inform Athena's Saints of Poseidon's challenge, in Rozan GoRouHou. There she found Libra Roushi, Kiki and the Bronze Saints. After informing them of Athena's status, Thetis quickly disappeared in Rozan's waterfall. Donning their repaired Bronze Cloths, the Saints followed Kiki, who used his heightened sense of smell to track Thetis down.

In the anime, the entrance to Poseidon's Temple was in Asgard (which doesn't exist in the manga), and Seiya and Shun were the first ones to enter, followed later by Shiryu, Kiki and Hyoga.

Later, Pegasus Seiya and Andromeda Shun found her in Poseidon's Undersea Temple with SeaDragon Kanon. He told her to explain the Saints what they were expected to do if they wanted to save Athena. She explained that Poseidon's Temple was supported by Seven Giant Pillars named Mammoth Pillar and that each one also supported one of the seven seas of the world, and that each was guarded by a powerful Marina General. If the Saints wanted to save Athena and stop Poseidon's flooding rains, they would have to defeat each Marina and destroy each one of the Pillars, and then the MainBread Winner, where Athena was being held.

Seiya and Shun told Thetis to lead the way, to which she replied, "If you let a mermaid lead you, then you know that death is what follows." The Saints agreed without hesitation, and she fought them briefly. (in the anime)

Some time later, Ophiuchus Shaina and Kiki had been sent by Libra Dohko to deliver the mighty Libra Gold Cloth to the Saints so they could demolish the pillars. Thetis tried to block the way, fighting briefly with Kiki, and then she had to engage Ophiuchus Shaina in battle, so Kiki could escape, since there was no time to waste.

In the anime, Shaina is the only one who is sent with Libra Cloth, and when she met Tethis, she entrusts Kiki with the Libra Cloth in order to carry it to the Mammoth Pillars so the Saints can shatter them.

Thetis met her defeat at Shaina's hands, but she didn't die. Since she was very faithful to Poseidon, she remained in the Temple in case she could still be useful to him.

In the manga, after Poseidon is defeated and his soul locked away once again, Julian Solo returns to his senses and decides to travel the world with Siren Sorrento to help those in need. He finds in the seashore a beautiful coral-colored fish, and remembers that long ago, as a little boy, he once saved a fish of the same species from dying out of the water. That fish he saved back then turned out to be Thetis, who was indeed a mermaid.

In the anime, she is last shown as the person who saves Julian from dying in the collapse of the Undersea Temple, after Poseidon's soul has been trapped. She takes him out of the water, and then she returns to the sea, no longer fully as a human woman, but as a mermaid.

Scales
The Mermaid Scales represent a Mermaid, a half-woman, half-fish being found in the folklore of several cultures, that would enchant sailors with their beautiful voices, and then kill them.

Thanks to Kurumada's non-complicated design of the Mermaid Scales in his manga, it wasn't needed to modify it in the anime, to make animation easier.

[Notes

 * Masami Kurumada named Thetis after the character of the same name in Greek mythology. The mythical Thetis was a Nereid, and was mother of the greatest Greek hero, Achilles.
 * Unlike the female Saints, Poseidon's female servants had no need to cover their faces with masks.
 * Masami Kurumada made Denmark her country of origin and her love for Julian as a reference to Hans Christian Andersen's The Little Mermaid tale.
 * Though she didn't have a Mammoth Pillar to guard, she was also considered a Marina.
 * In the anime adaptation, the events that involve Thetis are identical to the manga. There is only one minor modification: In the manga, she was last shown by Kurumada as the fish Julian finds at the seashore. In the anime, she is last shown delivering Julian back to safety, (which she also did in the manga) and returning to the sea as a mermaid.