We all know that his fatal flaw is loyalty. I've seen ppl say how they don't see it's a flaw. I get it but it is a flaw. It almost kills him. Percy's flaw is what drags him into Tartarus. He can't let go of Annabeth. He knew that the quest was important and he could have let her go but he didn't. His loyalty is the reason he fell into Tartarus. I know this isn't much but it shows that his fatal flaw of personal loyalty is actually quite fatal indeed.