One of the greatest Greek warriors, who was dipped into the River Styx in the Underworld as a baby by his mother. When he was out in the Styx, he was held by his heel so it remained his only vulnerable point. Helen shot him with an arrow at his weak point (his heel).
The term "Achilles' heel" is originated from this story. He was mentioned several times in the Percy Jackson Series. He appeared as a ghost in the The Last Olympian to warn Percy before he bathe in the Styx.