Arrietty and the Boy
These two start off on the wrong foot in a major way. Each is prejudiced against the other for being either a human or borrower. Both, however, learn from each other about how the other half lives, and become friends.
After all, they have a lot in common. Both have been cooped up and deprived of independence, and both relish the opportunity to learn new worlds together. Both of them want a friend, and they're willing to endure a fair amount of trouble to achieve that goal.