Fatal Flaw

A fatal flaw is some kind of weakness, mentally or physically, that all heroes and half-bloods, but other beings can have one, such as humans and immortals. These flaws can often be the downfall of any being. The topic of fatal flaws, first shows up in The Sea of Monsters when Annabeth says listening to the Siren's song will allow you to learn (normally about your fatal flaw). She then explains to Percy about fatal flaws and the topic appears over the rest of the series.Percy's flaw, loyalty, is told to him by Athena. She says it is a dangerous flaw for the hero of the Great Prophecy, possibly because it means he would "sacrifice the world" to save someone he really cares about. Athena also says that flaws that have a good motive are usually the most dangerous ones.

Siren's Music
The Siren's music has an oral hypnotic effect. When sirens sing, they sometimes reveal your fatal flaw. Flaws can produce an enticing image which is what lures sailors and adventurers to their deaths, either by drowning, being eaten by the Sirens, or by hitting the rocks on the island. When Annabeth and Percy are passing by their island, she asks him to tie her to the mast so she can ear the song but not escape to her death. However, after hearing the song, she manages to set herself free and dove overboard because of the Siren's music hypnotising affect. When Percy goes to save her, he sees her vision. She thinks she can fix Luke and bring her mother and father back together, essentially. This is due to her fatal flaw, hubris (pride).

Known Fatal Flaws

 * Percy Jackson's fatal flaw is excessive personal loyalty. He will risk his life for anyone; his friends, family, strangers, and sometimes even his enemies (like Clarisse when her dad was bullying her).
 * Annabeth Chase's fatal flaw is hubris (deadly pride). She thinks she can do anything better than everyone even the gods, such as saving her family (seen in The Sea of Monsters when she heard the Siren's music).
 * Nico di Angelo's fatal flaw is holding grudges. He will hold a grudge even if it costs him his own life. In The Battle of the Labyrinth, his deceased sister, Bianca, tells him that holding grudges are very dangerous for the children of Hades.
 * Bianca di Angelo's fatal flaw is currently unknown, but it can be assumed that she often holds grudges like Nico because she is a child of Hades.
 * Thalia Grace's fatal flaw is ambition. She desires power even though she knows she does not need it.
 * Jason Grace's fatal flaw is a deep sense of duty for those he cares about. Jason feels a need to do well and not let his friends down. He also has a strong moral sense of right and wrong.
 * Leo Valdez's fatal flaw is, as of yet, unconfirmed. However, readers have observed his feelings of inferiority, thinking that he has to prove himself constantly and that he is lesser than anyone else, like Jason Grace.
 * Piper McLean's fatal flaw is currently unknown, but readers have seen that she occasionally suffers from lack of self-confidence. She thinks that she is not powerful enough to do anything, and she thinks she's not perfect.
 * Drew's fatal flaw is vanity (excessive pride in ones appearance, qualities, abilities, achievements, etc). This flaw is most likely shared by many children of Aphrodite.