The goddess Thetis is the mother of Achilles. The main theme of Homer's Iliad is "the wrath of Achilles" who withdraws from fighting with the Achaeans against the Trojans due to feeling insulted about Agamemnon having taken his war prize. The reason that Achilles' withdrawing from the fighting is so important is that he is the greatest of warriors on the Achaean side. When Achilles was a baby, Thetis dipped him in the River Styx to make him invulnerable; he could only be injured in the heel by which she held him.
One of the major events that caused the Trojan war was Eris throwing an apple of discord into the wedding party of Thetis before Achilles was born. Thetis also has Hephaestus make Achilles his famous armor and intervenes with Zeus on behalf of Achilles during crucial plot events.