Hobbes certainly appears real to Calvin, but he is a stuffed toy.
And as Hobbes is a total figment of Calvin’s imagination, Calvin could gentrify him at any time of his choosing. Make him an example of learning, sophistication and fun. But at his current age and stage of development Calvin choses not to. Calvin is the worry, it’s not Hobbes who is leading him astray.
He is incredibly violent. Literally every school day he slams into Calvin at a horrifying velocity. He frequently attacks or mauls Calvin in the house for absolutely no good reason, and even tried to eat him once.
It’s illuminating that in their brutish, slapstick relationship Calvin never causes damage to Hobbes. And the only time Calvin actually gets hurt is when he does it to himself.