If you add a hen to a rooster they will take a few weeks to get used to each other.
The rooster will probably be happy to see a hen immediately, but he could be defensive at first if he is well used to being alone and/or has not been with hens before. He should come around to her quite quickly though.
At first the hen will not accept the advances of the rooster and will need enough space to escape him. He may become aggressive with her which is not good. A good rooster will keep trying to woo her with tidbitting and wing dancing and eventually she will learn he is good for her to have around and submit to being mated.
However, after they have accepted each other, a rooster with only one hen could end up overmating her which could lead to her losing feathers and even being injured. It all depends on the temperaments of the hen and rooster. Some roos are fine with only one hen, some need more.
Also there is a small possibility of the rooster becoming more human aggressive once he and his hen have established their relationship. Roosters have strong instincts to protect their hens and sometimes consider humans a threat to them, for example when someone tries to grab a hen.
At the end of the day roosters and hens are made to live together so it will probably work out ok and is definitely worth trying, but be prepared for potential problems.