This was originally going to be a short straight-to-video project, but Toshio Suzuki was so impressed with Hiroyuki Morita's storyboard, and especially the main character of "Haru", that he encouraged Hayao Miyazaki to give this a cinematic release.
While every other cat in both the human world and the Cat Kingdom has paws, the Baron has human -shaped hands, covered in gloves and sporting long fingers and visible thumbs. He is also noticeably smaller in the human world, about two -third the size of an average cat. These attributes mirror those seen on the Baron figurine in Whisper of the Heart.
The trivia item below may give away important plot points.
The English-language title of this movie is "The Cat Returns". There are several interpretations of this title. The character of the Baron is recurrent: he originally appeared in Mimi wo sumaseba (1995) as a statue, but was nonetheless one of the main plot-points of the film. The title could also be a reference to the notorious fish-thief Muta alias Renaldo Moon who has returned to the cat kingdom after many years (and he also originally appeared in Whisper of the Heart). Another possible explanation, is the return of the cat Prince at the end of the feature, to set everything right. Another possibility is Yuki, the "dirty kitten" that Haru helped as a child, and "returns" to save Haru, as Haru has saved her.