I am a Rick Riordan fan. I loved the Percy Jackson series. In fact, I prefer Percy to Harry Potter. Percy is cute, strong, brave, and decidedly not self pitying, unlike Harry. Percy doesn't whine. Nor does Jason from the Heroes of Olympus. He's buff and honorable, and all around appealing character. I am not quite as big of fan of the Kane Chronicles, just because I don't particularly care for the focus on death in Egyptian mythology, but Carter and Sadie are appealing characters as well. All of these heroes and heroines are flawed in unique and different ways, and they are all sympathetic to readers.
On the other hand, Apollo is a mess. His voice, despite his body and his deteriorating memories, is clearly adult. He is a messed up dude and a total narcissist. I wonder how children will relate to him. In addition, a reader has to have more than a passing familiarity with the loves of his immortal life. Apollo is being punished by Zeus by being forced to live as a limited, unattractive human teenager. Through his experiences, Apollo begins to grow in maturity and insight. It is a clever trope and very successful in the story. Apollo does become more appealing as the story proceeds. And yet, I wonder if kids will relate to his adult attitudes and reflections. I'll have to ask my students. I really enjoyed following the travails of this very flawed god.