Luke Castellan
Luke dies in The Last Olympian when he is given Annabeth's knife by Percy. He sacrifices himself to defeat Kronos by stabbing himself under his left arm where his mortality was still kept intact after his bath in the River Styx (known as his Achilles heel). As he dies, he asks Percy to make sure all the demigods are claimed to avoid this happening again which might mean that they had become friends again because he is forgiven by Percy. Although he is seen as a villain throughout the series, in the end he is treated like a hero and forgiven by everyone. He is also the hero mentioned by the prophecy of the Oracle in The Last Olympian.