Paradigm.

In, Charlotte's web, White uses some Archetypes to create a complex, story filled with depth. 
Heroes; this character is courageous, Nobel and is changed in some way by the story. I know this is not Fern because she isn't changed by the story, she ends up becoming a "normal" girl and eventually runs off with Henry Fussy, its not anyone of the barn animals, like Templeton or the goose. it could be Charlotte because she ends up doing something for herself, i guess she didn't exactly have much of a choice though. the most obvious hero would be Wilbur because when Charlotte finally dies, he actually showes some mournful feeling, and appreciation for Charlotte. it is the first time in the book he shows feeling for anyone else (except were he warms up to Charlotte and fern stops visiting the barn as often.)
Mentor; this character assists the hero in some way and expressed experience and knowledge. it seems like this character would be Charlotte's seeming as she is always teaching Wilbur new things and it sorta like a mother figure to him after Fern isn't as interested in him as she used to be. But at the same time, a less obvious Mentor would be the old lamb because he is also teaching farm life to the animals and helping Wilbur along in the story. The Dr was also a mentor in Charlotte's web, but he was also a microphone for White so, maybe that doesn't count.
Shadow; i think the Shadow in this book was death and loneliness, it was basically just hanging around every bodies head during the entire book. it created tension, and it defiantly represented the "darker side of nature"
Shape shifter; i think Templeton was the shape shifter in this book because he was always uncertain, and kept changing his mind about things like weather he wanted Wilbur to die or not. weather he cared or not about people. but a better shape shifter would probably be spring time, and the hole concept of things being born and change and life. i don't know, it all sounds very philosophical.
spring can't be uncertain or change its mind. Ms Arabel can, and Avery can.