Who saved Andromeda from the sea monster

Perseus was returning from having slain the Gorgon Medusa.

After he happened upon the chained Andromeda, he approached Cetus while invisible (for he was wearing Hades's helm), and killed the sea monster.

He set Andromeda free, and married her in spite of her having been previously promised to her uncle Phineus.

At the wedding a quarrel took place between the rivals and Phineus was turned to stone by the sight of the Gorgon's head

Read more...https://en.wikipedia.org/wiki/Andromeda_(mythology)