Moon is a plump cat appeared in Whisper of the Heart. He is Shirō Nishi's pet, technically, but he often runs away and drifts from house to house, being called by many names. He's a street cat and doesn't stay in one place. He leads Shizuku Tsukishima to Shiro's shop, Chikyuuya.
Moon also appears in the film The Cat Returns. Here, he is called Muta, but has the same free spirit.