Lefty, I honestly like your troublemaker cousin idea the best. My only idea was to have a twin or something, but that would change a lot of things and bleah.
Remember this option, too, if we're just going to kill off this new character: why don't we just keep Hannah and kill her off right away? It'll keep things simpler, but still add a realistic aspect and a bit of terror. Her death can be the beginning of the conflict, when the other characters begin realizing something is wrong. The morning after the field trip, Hannah can be found dead in her bedroom, with contorted features and a baboon tail or something. That'll show that the chip and the technology is far from perfect, and it'll kinda alert the government that probably some other animorphers are on the loose. At her funeral, the eleven students (or those that come) could be making eye contact and planning what they should do.
