I think it would be fun for Vyvian and Caelee to have a bit of a rivalry. They both shoot bows, but Vyvian has more natural talent with sneak attack and a higher dex. Vyvian is one year older but inches shorter, creating an interesting "big sister, little sister" dynamic, and they both sneak out of their rooms at night. When one gets caught and the other doesn't, that would probably breed a little resentment. Seems like the perfect person for Caelee to argue with about who is more mature (of course that shows that Caelee is NOT as mature, but kids don't think that way).