Achilles
Later stories depict Achilles as being invulnerable in all of his body except for his heel, as that is where his mother Thetis held him when she dipped him in the River Styx as an infant.  As such, the term "Achilles' heel" has come to mean a point of fatal weakness.  Because of that, his heel was still vulnerable.  However, the Iliad does not say this.