Friendship stories are a mainstay of PBs. The one that comes to mind immediately for characters equal importance to the story is Holly Keller's Farfallina and Marcel. They aren't children but it could serve as a mentor text. Same with some of the chapter books like Frog and Toad, Fox and Chick, Bobo and Pup-Pup. I hope this is helpful even though it's not exactly what you were looking for. Good luck.