Did you add her in at night? I always slip my hens back in at night.
All my hens run from Jack the roo,but he is not super mean.Not missing feathers or any sort of damage,but at first glance I was thinking he WAS mean.It just looks bad,but turned out it was pretty normal.If he is hurting her can you put a saddle on her to protect her?
I got my roo when he was a year old.He does now what he did then(year ago).If there is no actual harm done then it would seem to be normal.
Only other thing I can think of is that maybe he targets that hen because she is easy to catch.Our roo will chase down whoever he can when the mood for sex strikes him. I don't think he targets one over another....just whoever is close by when the urge hits him.Sometimes he gets them and sometimes not,but he sure tries!