Achilles had one mortal parent and one parent that was a goddess. His father was a mortal and his name was Peleus. His mother was a sea goddess. Thetis his mother was only a minor sea goddess and not well known. Patrokles, Achilles cousin fought in the war with him. Patrokles stole Achilles armor to try to kill Hector, but he failed and was killed by Hector. This is why Achilles killed Hector. Thetis was admired by Zeus but rejected his advances. Then, Themis revealed that if Zeus was to have a kid with Thetis then their child would over throw him and be much stronger. Nereus was Thetis' father. Hephaestus was Achilles step brother they had the same person raise them, Thetis. Hephaestus' real parents were Zeus and Hera.