Aaron was the priest of Prentisstown, and the main antagonist of the first book. He is insane and continues to pursue Todd and Viola across New World. He wants Todd to kill him so that all the boys of Prentisstown become men. Aaron also believes that 'if one of us falls, we all fall'. This could be interpreted as: if Todd does not kill Aaron to become a man, Prentisstown and its ideology will have been unable to manipulate Todd (The last boy) and thus exist uncompleted (probably).
In the book Edit
At the beginning of the Knife of Never Letting Go, he hits Todd when he swears. When Todd tells Ben, he says that Aaron is crazy. When Viola first landed, Aaron found her and thought she was from God. He is the man who Todd was to kill when he became a man, and Ben was originally what they were going to use to persuade him into murdering Aaron, but Aaron continuously tries to track down Viola so they can use her as the sacrifice instead. When Todd first runs away from Prentisstown, Aaron chases him but his face gets mauled by a crocodile and is left behind.
After he kidnaps Viola from the Spackle's camp, Todd plans to rescue her and devises a plan. Unfortunately, Aaron captures Manchee and tries to exchange the dog for Viola but Todd refuses. Aaron kills Manchee as Todd and Viola watch.
On their way to Haven, Todd and Viola hide in a church under a waterfall where Aaron finds them and reveals the plan about him being the "sacrifice" and how he wants to be a saint. He tries to get Todd to kill him by fueling his Noise but he won't give in so eventually, Viola is the one who kills him.