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 never seen as anything more than a stuffed rabbit, having no "alternate reality" of his own (at least in Calvin's eyes). Susie can't understand how Calvin plays with his "dumb ol' tiger, but not Mr. Bun". However, Hobbes, in his "live" form, has called Mr. Bun kind of comatose. 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 could possibly 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".