Funny, I was just flipping through my copy the other day, and the cast list for 'Blood of Olympus' is surprisingly huge, even in graphic novel form. Honestly, it gets a bit messy trying to juggle them all on the page sometimes. You've got your two main groups: Jason, Piper, and Leo are trying to stop Gaea from waking up, while Percy, Annabeth, Frank, and Hazel are trying to get the Athena Parthenos back to Camp Half-Blood to end the Greek/Roman feud. Then Nico and Reyna pop up with their own crucial quest to haul that statue across the Atlantic, which honestly might be the most tense plotline for me.
Oh, and you can't forget Coach Hedge. He's technically a main character? More like comic relief with a baseball bat, but I love him. Gaea and the giants are the big bads, of course, but they feel a bit underdeveloped in this format compared to the novels. The focus is really on the seven demigods plus Nico and Reyna, trying to tie up their individual story threads before the final battle. My only gripe is that the graphic novel, by necessity, has to trim so much inner monologue that some characters, like Frank or Hazel, feel a little less fleshed out than they did in the original.