Fly was the Musca saint of the Century XXI. He and Michelangelo never were loyal to Atena or to Mars.
Fly is a selfish guy that got a silver cloth after offers to Mars various clothes of saints that fought against Mars. He loves clothes, and love his cute silver cloth, so, he becomes a Mars servant and continued offers clothes to Mars, and the more beautiful, the better. He was sent to the Wind Ruin to protect it and his and tries catches the clothes of Yuna and Kouga, and after defeats Yuna, Kouga breaks his precious silver cloth, and Fly, how he is not an Athena servant, probably gets away and never returns to serves Athena against Pallas.