Mr. Bun is the stuffed rabbit belonging to Susie Derkins and who frequents Susie's "tea parties" as a "guest." Unlike Hobbes, Mr. Bun is always seen as nothing more than a stuffed rabbit, having no "alternate reality" of his own (at least in Calvin's eyes). In a week-long series revolving around the loss of the stuffed Hobbes, who is found by Susie while out for a walk, Calvin loses the companionship of the alternate Hobbes while the actual stuffed and motionless Hobbes is served tea alongside an equally motionless Mr. Bun. The alternate Hobbes reappears when Calvin finds him sipping tea alongside Susie and Mr. Bun.
Given one of the more unusual aspects of the Calvin and Hobbes universe, namely the fact that the only other children in the strip besides Calvin are Susie and Moe, Mr. Bun may well be Susie's only friend. This is evidenced in one strip where Susie invites Calvin to tea. Calvin, true to form, loudly refuses. Susie tries to pretend that Calvin's rejection was not worth worrying over, offering to refill Mr. Bun's teacup while commenting on the rudeness of "Mr. Calvin." The final panel shows a dejected Susie, head in hands, with her final word on the situation: "Poop."
See also: Hobbes' reality