A loyal servant of Poseidon, bound to him by a debt of life. Thetis was saved by Julian Solo when he was a little boy. She was then a beautiful fish, stranded on a beach, and Julian tossed her back into the sea. During Poseidon's kidnapping of Athena, she served as mediator between the factions of Poseidon and Athena. Thetis is not one of the Mariner Generals, but a middle-ranking soldier, as she is superior to the Mariner rankless soldiers, but subordinate to the Mariner Generals. She was unable to assist the Mariner Generals due to her defeat by Ophiuchus Shaina. The Saint spared her life and Thetis ended rescuing Julian Solo from the collapse of the Underwater Sanctuary after Athena removed Poseidon's soul from him. Thetis turned back into a fish as she died and Julian, upon finding it, let it back into the ocean, finally laying her to rest.