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 mariner of low rank and if the generals are "counterparts" to the gold saints, she may be the counterpart of a silver saint. It's unknown if other mariners of the same rank ever existed in Poseidon's army of if she is one of a kind.
Saints Group : poseidon