Animated (anime?) movie with a boy who is transported to another world by a girl, accidentally. In this world he meets his backpack who is now a cute living creature and learns that he can become anything he can imagine. I think the girl wanted to transport another boy she had an eye on to help and save her world/kingdom from a great threat, but this other boy got summoned instead. He vowed to save it nonetheless. The threat moved to the real world.
The Toy Warrior (2005)
The plot synopsis on IMDb matches your description quite well. It mentions saving the world, imagination and the loyal backpack.
The Land Of Toys is a world of pure imagination, ruled by the popular doll Princess Sherbet, but when the mysterious Dark One threatens Sherbet's toy kingdom, she turns to the real world in search of a heroic Warrior Prince... Unfortunately, she ends up with the imaginative trouble-maker Jinoo. When Jinoo accidentally touches Sherbet's magical Warrior Stone, the spunky sixth-grader is suddenly transported to the Land Of Toys to become the powerful Toy Warrior! Aided by the reluctant Princess Sherbet, the transforming Police-bot, R.J., and Jinoo's amazingly loyal backpack, Ping, Jinoo is ready to play Toy Warrior and chase the dark forces across the landscape of imagination. The evil is spreading and now both the real world and the very Well of Imagination itself are at stake! Our only hope is a hero with heart... the Toy Warrior Jinoo!
It is available online at YouTube but in Hindi. From skimming the video the below appears to be the boy's backpack as a creature in the new land.
I found this by Googling the new title of the post but specifying it must include backpack:
Animated (anime?) movie with a boy who is transported to another world and meets his "backpack" as a living creature. This then gave me a Movies & TV question and answer about the work back from when they accepted story identification questions.