User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5604127-20130302213549/@comment-3146930-20130306011001

However, using your own example of Silena and Clarisse, we only learn that the two are friends during The Last Olympian, the last book in the series. Before that, Silena only had small cameo roles, with her first big role being in The Demigod Files. Clarisse on the other hand is just a bully in The Lightning Thief and didn't get any kind of character developement until The Sea of Monster before pretty much vanishing in TTC, then has a small role in BotL. The Heroes of Olympus has only three books so far, leaving plenty of room to learn about other characters.

Comparing book for book, in TLT, we learn a lot about Percy, Annabeth, and Grover. There are a few other important characters like Mr. D and Chiron, but most of the other characters are easily passed over because they aren't really the focus. In TLH, we meet three new demigods, much like TLT, but we also learn a lot about Coach Hedge. We do get to see a few old characters that pretty much do the same job as they did in TLT, moving the story along as needed.

In the SoN, we meet for the first time another camp of demigods and learn a lot about Reyna and Octavian, as well as having Hazel and Frank introduced. The SoM introduced Tyson and we learned a lot about Clarisse, but that doesn't work in SoN as we are meeting the people at Camp Jupiter for the first time. SoN was in a lot of ways an introduction book, bringing in a lot of characters that may or may not be more important later.

The Titan's Curse was also an introduction book, bringing in Thalia, Zoe, Bianca, and Nico namely, but we really don't learn that much about Nico, Bianca dies halfway into the book, Zoe dies as well by the end, so the only character we really learn a lot about is Thalia. In a way, this is kind of where The Mark of Athena failed, as we aren't really introduced to any new characters. However, we do learn a lot about the core seven demigods and how they react to each other. We get to see how they all interact with each other, their strengths and weaknesses, what they all think of the others, and I feel that it makes up in lack of characters by learning more about the ones we have. Plus, Jason and Percy have their memory back so we really get to see them open up, something we didn't really see to much of in the first two books.

This is just my opinion however.