In Hero stories of Greek mythology, every hero has a fatal flaw. Same goes for the heroes in Rick Riordan's stories. My characters have fatal flaws as well.

Affinity Darwin (Child of Hades) - Curiosity
Affinity should be the poster child for "curiosity killed the cat". Even as she learned the mind-blowing truth of her true heritage, while she was full of denial, she was also full of questions. Her ability to feel others emotions also ties into this flaw- it's like seeing a glimpse into someone's mind, and she wants to know more. Her curiosity pushes her to pursue Mico, even though they are beyond reasoning, enter dangerous quests willingly when she is so underprepared, and why she is so willing to participate in dangerous endeavors, such as traveling to the Underworld

Sol Andrews (Child of Apollo) - Excessive Loyalty
Sol is willing to lay it all on the line, even at the risk of his own life. He is willing to help anyone with anything- a sibling in need of relationship advise, a fellow camper who is injured, to bigger things like throwing himself in the middle of a one-on-one between Affinity and her enemy. It leaves him prone to predictability by his enemies, on top of the dangers that come along with throwing himself into dangers that he may not be prepared for to save a friend

Mico Becerra (Child of Zeus) - Holding grudges and feeling of inferiority
Mico has two fatal flaws- holding grudges, even at the risk of their own life (for example, even though they're aware that they're expendable to Ouranos, the hatred they feel for their father is too insurmountable to get over, and they want revenge) and a feeling of inferiority (even though they're the child of Zeus and a very powerful demigod, because they were never claimed, they feel inferior and inadequate compared to other demigods, like Affinity and Sol, who were)

Quinn Crestfall (Child of Ares) - Hubris (deadly pride)
Quinn's father has drilled it in his head that being a child of Ares is the greatest gift he could have ever received. As ridiculous as this claim seems, after years of hearing it from his father and his siblings in the Ares cabin, it became groupthink brainwashing that he struggles to shake. He believes he is the strongest, fastest, most strategic, smartest, and just genuinely a better individual and hero than those around him, and this can both lead him to jump into situations that are beyond his skill level, and make enemies where there could have been allies