First off, this is one of my favorite books in this world (including pjo). I loved seeing the characters grow and develop, especially Percy and Nico. This book was also the one that made me like Piper and Jason (I'm ashamed to say I wasn't a fan before HoH). Anyway, back to character development.
From book one, Percy has had this dark side that occasionally makes little surfaces. However, it is here that it fully develops and contrasts with his power. I love seeing how much he has changed since he was a little kid. In this moment, you see just how much power he has and what he could do with it and he sees it to. It is a testament to his loyalty and character when he stands down.
Then Nico. To be honest, I saw it a mile away. Of course, that doesn't stop me from wanting to give that angsty boy a hug. :P Nico's development really kicks off again from here (and into the next book). We see how much he has changed from the cheerful boy Percy met in book 3 of the original series. It is heart wrenching, especially when you read over any of their scenes after his revelation.
The other characters:
Jason- Give this boy a cookie. I am so proud of him. Agh. Just the way he's acted with a ton of maturity throughout the entire book made me rally behind him. Jason Grace is bae.
Leo- Oh, baby. Sweetie. This kid's lack of confidence hurts me. This kid has gone through the miller, but he still stands tall. He is a very strong character too. He's been developing a lot. However, I'm not a fan of the whole *SPOILER* Calypso thing. It was nice to have at least one character not paired up and everything, and Leo's lack of a romantic partner was really nice, personally speaking. However, he did become more focused and serious, so that's a plus.
Hazel- THIS GIRL. This. Girl. She has become so strong and agh. Her growth in this book was amazing as well and I'm so happy to see her come into her own.
Frank- Okay, with him I'm a little less pleased. While I'm happy he became more confident and everything, the whole *SPOILER* physical change thing was not necessary. Not every hero has to be perfectly fit and handsome. I related to this goofball so much with that. I have a childish face and a muscular/chubby body too, so seeing a character like that be a hero made me feel a lot better about myself. I'm still a little upset about that, but everything else....go Frank. You go boy, defeating baddies and taking names. :)
Annabeth- alsdkfjalsdfkjasdlkfj;. This girl. She could punch me in the face and I'd say thank you. She is why they survived Tartarus. Without her, they would not have made it. She is such a strong character and I love her for it.
Finally...my response to the homophobic reviewers (warning for minor swearing):
Gay is not a dirty word. It is not something to shield your children from and cringe away from. There are children- maybe even your own- who feel broken and alone because people like you are to stuck up your own ass to realize that they are Just. Like. You. So what if they love someone of their own gender? If they are attracted to someone like them? They can't help it, and you will only push them into self-hatred and destructive behavior if you can't accept them. You could lose your child. Kids die because you are incapable of the simplest maturity that elementary students who are educated are capable of. Rating books meant for this age group higher only because of one character's sexuality is alienating children from a figure they can relate to. Representation is one of the most important things entertainment can do, and if it means not telling homophobic assholes like you about gay or other LGBTQ characters, then so be it. The world is changing and leaving you and your outdated, ridiculous ideas behind. Already bisexual and transgender characters are entering the media. Steven's Universe, Avatar: The Legend of Korra, Adventure Time, Archer, How To Train Your Dragon, possibly Frozen, ParaNorman, Glee, Hey Arnold, Harry Potter (technically), Sailor Moon, and yes, Percy Jackson, all are mediums where LGBTQ characters reside. So shove your prejudiced beliefs up your ass and get educated. You can do it. I promise.
And I swear you will be happier with yourself that you would otherwise.
Please. For yourself, for your children, for the world.
And that begins with you.