Man has all the mammal characteristics, every one of them. So do whales. (OK, most whales lack hair. However, there are other naked mammals, especially aquatic ones.) A whale, like man, is a mammal and an animal.
You probably meant to say that a whale can have some fish characteristics but not be a fish. So it does. It has a streamlined body. But even stated correctly, all you have is a bad analogy. A whale lacks several salient characteristics of fish. No gills, no scales, no exothermy. Live birth rather than hatching from eggs. It suckles its young.
Now more than ever, we can tell what a thing is. Mammals are animals. Humans are mammals.
I was on a whale watch cruise off Cape Cod a number of years ago. This whale came right up next to the ship, and I could see **big** hairs around its blow hole. Big, like as thick as a pencil.
The naturalist on the cruise had passed around baleen and whale ear bones. The bones were the size of a softball!