Peter is loyal to Wendy because he is in love with her. It shocks Wendy that he would protect her; even after she severely wounded him for going against her.
The reason Leon is chosen for this mission is due to his soft spot for children. The vampire corpse that was found was a child, so the AX needed someone other than Abel to be merciful if needed.
Peter is mentioned to be a failure test subject. But it's never stated why. Maybe he's still human or just a normal vampire.
Abel asks for 13 sugars with his tea due to his constant hunger issues.
The entire premise is a twist on the Peter Pan story.