You are a werecat and Harry is a weredog, you turn into a kitten when the cat part of your brain gets stimulated. He can turns into a puppy when the same thing happens to him.

He carries you around in his schoolbag which has a little compartment to hold you that has some treats, some milk and some toys to occupy you while he's in class

You and Harry are in bed, it's nearly morning and he subconsciously pets your head, causing you to shift.

"Oh, look at the little kitty! I didn't mean to!"