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 Generals. She is a Marina of lower rank than the Generals and as they are "counterparts" to the Gold Saints, she may be the counterpart of a Silver Saint. It's unknown if other Marinas of the same rank ever existed in Poseidon's army or if she is one of a kind.

Thetis, the Mermaid of Poseidon
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 wipe humankind off the Earth for their countless sins 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 at the Lushan Waterfalls. There she found Libra Doko, Kiki and the Bronze Saints. After informing them of Athena's status, Thetis quickly disappeared in the Lushan Waterfalls. 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 Sea Dragon 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 Pillars 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 Main Breadwinner, where Athena was being held.

Some time later, Ophiuchus Shaina and Kiki had been sent by Libra Doko 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 Ophiuchus Shaina engaged her in combat so Kiki could escape, since there was no time to waste. In the anime, Shaina is the only one who is sent with the Libra Cloth, and when she met Thetis, 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 was not killed. 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 Scale 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 or take their souls. The most famous of mermaids is Hans Christian Anderson's Little Mermaid, who, in contrast to her legendary counterparts, came to love a human and wished to acquire a soul of her own through love. 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.

Skills and abilities

 * Intuition: As a being whose Cosmo has awakened, Thetis can manipulate the sixth sense.
 * Transformation: By achieving the sixth sense, Thetis was able to assume a humanoid form.
 * Superhuman Strength: Like most Cosmo users, Thetis' physical strength surpasses that of average humans.
 * Superhuman Speed: Like most Cosmo users, Thetis' physical speed surpasses that of average humans.
 * Coral Manipulation: Thetis can manipulate very resilient, living particles of death coral, which can be used to immobilize and asphyxiate the opponent. Shown in her Deathtrap Coral technique, which only exists in the animation.

Secret Techniques
Deathtrap Coral (デストラップ・コーラル, Desutorappu Kōraru): Thetis manipulates the coral around her to ensnare and then asphyxiate her foes.

Trivia

 * Masami Kurumada named Thetis after the character of the same name in Greek mythology. The mythical Thetis was a Naiad, and was mother of the Greek hero, Achilles. To protect her son after learning he would die young, she dipped Achilles into the river Styx as a baby, which made his body invincible, except for his heel (since she held that in order to dip him in). She was also the adoptive mother of the god Hephaestus after his parents, Zeus and Hera, rejected him and threw him off Olympus.
 * 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.
 * Not being a General, but a lower-rank Marina, she didn't have a Mammoth Pillar to guard.
 * 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.

Quotes
"If you let a mermaid lead you, then you know that death is what follows."