I too have never heard of a broker in North America for rentals, but having lived in Chile and Australia, brokers run the markets there, and are universally hated for being lazy and not worth the money they take.
And if that's not enough, in Japan there's no law that regulates the rental contracts so, every time that someone wants to rent a flat, he has sign a bunch of abusive clauses.
In any case, being a broker must be a good business because Tokyo is full of them.