The male is probably trying to mate with her and she isn't having it. Ideally, you need at least four females to every male. Then he won't bother only one female, and eventually they will squat down for him when he comes close instead of running away.
You could get a few more females and try to integrate them into the flock. However, you always need to be careful when trying to introduce new birds because the two you have now will be very territorial. After you got new hens, you could move all of the birds into a new cage, or if you don't have that you could try to totally rearrange the old cage so that it feels new to your current birds.
For now, you can make sure she has lots of cover to hide from him. You might see that she doesn't lay because she's stressed out.
If he is chattering and growling at her and pecking at her face, he is being aggressive and you should separate them.
Sometimes birds decide they don't like each other. I have a female who almost killed the male she and some other hens were with. I removed the male to let him recover and then tried to reintroduce him but she went after him again.