It's true she was at one point more powerful, but even then she was bound to her grove - she had limits, rules she had to follow. And despite her centuries of life, she still had the nievete to trust him. And compassion is normal emotion, and her primary one. She acts like a very nice human. Bard doesn't even bother with/is incapable of communication.
Also, balance isn't his goal.
When the unknowable structure of the universe is threatened, Bard steers all existence away from utter annihilation.
He at least is working towards preserving existence, and balance is probably the means to the end. Of course, we don't know if he's just doing it out of the kindness of his heart, if it's his duty, or if he even really thinks. He may well just be more like a part of a great machine, simply doing what he does, not putting any conscious though into anything. A body with no will.
I'd guess it's somewhere between those.