Achilles is known to be an immortal warrior and the greatest of all of Greeks. But his only weakness is what makes him penetrable. That weakness is his heel. The character Superman is quite similar in this aspect, he is almost an immortal but like Achilles he also has only one weakness that would bring him down. That weakness is known as Kryptonite. But these heroes are immortal due to different reasons. Achilles is like this because while he was young, his mother Thetis held him by the heel and dipped him into the River Styx. The only reason why he isn’t completely immortal is because his heel was the spot where his mother held him, wasn’t touched by the water.