Mousses was the Cetus saint in XX Century. He doesn't shows stronge power, because Marin would win him easily, but Asterion helped him (and Seiya kill him even with Asterion's help).
Mousses was sent to kill Seiya in the anime and the manga, because he disrespect the Sanctuary law. But, in the manga, Misty attacks the Mount Fuji and kill Ikki (another sinner) and Babel, Asterion, Mousses and Marin was sent with him; but Mu teleports the black saints and Seiya, Shiryu, Hyoga and Shun and Marin guides the 3 conrades to kill the wrong victim; Babel kill the Black Cygnus, Asterion kill Black Andromeda and Mousses with Black Draco, and these black saints was confused with Hyoga, Shun and Shiryu; Marin simulates kill Seiya, and the 4 corpses are buried in the beach; but Misty ask to Marin, Mousses, Babel and Asterion.
After this, Mousses, Asterion and Marin came back to the beach to search for Babel and Misty, that don't returned to meet them. They find Babel and Misty deads with cards of Pegasus and Cygnus; but, how Hyoga and Seiya killed Misty and Mousses, if they was killed? Marin, that killed Seiya and guides Babel to kill Hyoga, must know. Babel read her mind, and discovery that she is a traitor; Marin fight against Mousses, but Asterion tell him the moves of Marin, and Mousses defeats she, and the two saints imprisions Marin in the sea, to makes she a hostage, to atract Seiya.
Seiya arrives in the beach, and Mousses almost defeat him, but when Seiya hear Mousses telling Marin is his lost sister (Asterion read this in Marin mind), Seiya create power to defeat Mousses, because Asterion can't notice quickly the moves of Seiya.