I think that Cheren will take battles seriously, and wouldn't hold back just because you are his friend. That's the impression I get from him.At one point, Cheren says that his objective is to become the Pokémon League Champion, so I daresay that he will end up being a formidable foe, particularly in the game's later stages.
Belle has been said to be forgetful, clumsy, and her father doesn't believe she should travel on her own. So initially, she will probably not be a challenge. But I have a feeling that at some point in the game that will change and she will become a formidable trainer, surpassing Cheren.