Usagi's constant companion. He is a young boy who looks to be between six to eight years old, although he seems somewhat more skilled in every day tasks than Usagi (In Volume 2 it is revealed that he even helps her dress). It is also revealed in Volume 5 that Usagi raised him after her dog, Shiro, found him when he was still a baby. Hanzo thinks that this is the root of his fear of dogs. Later on it seems that Mamezo's deepest wish is to have Usa as his mother and to have Hanzo as his father.