You could argue that all three of the Whites serve as the protagonist, but we're just going to go with Mr. White. Why? Because he's the one making the wishes. A protagonist isn't necessarily your favorite character, but the one who really drives the action, which is exactly what happens when Mr. White insists on buying the paw and using it to make three wishes. His last wish, in particular – the wish he makes all by himself – is really what makes him stand out in our minds as the protagonist.