Although the third-person point of view typically follows either Amy or Dan, it sometimes shifts to show what's going on with the other Cahills, like Irina Spasky or Alistair Oh. It's like how The Amazing Race shows all the different teams during the course of the race so you get an idea of who's teaming up with who, and what particular challenges each team is facing. It also gives us a closer look at their motivations and raises suspicions about who is loyal to whom.
But throughout the story, we stick closest to Amy and Dan. That means that, of course, we're rooting for them to find the clues and get to the next phase of their quest. It also means that it's easy to view all the other characters as straight-up villains, since they're the ones standing in Amy and Dan's way.