More specifically, Hobbes wasn’t meant to be explicitly imaginary. Elaborating on that, to Calvin, Hobbes is a walking, talking, anthropomorphic tiger. From the perspective of other characters, the readers see a stuffed tiger.
That might seem pretty open and shut, that the anthropomorphic Hobbes is just in Calvin’s imagination, but Bill Watterson, the creator, explains it a little differently.
He says that these two visuals, the live animal and the stuffed animal, are really a dichotomy, or a separation of two parts of one thing. “When Hobbes is a stuffed toy in one panel and alive in the next, I’m juxtaposing the “grown up” version of reality with Calvin’s version, and inviting the reader to decide which is truer,” said Watterson.
So what does it all mean? Basically, Hobbes is 100% real when he’s seen from Calvin’s perspective, and 100% unreal when seen from someone else’s. So rather than Hobbes just being “imaginary,” there is no real definition of his reality.