Exactly! The idea is that you can have a harem, and if you don't progress with girls before he does, he would "get them" first (Maybe they would like him most or something, I don't know yet xD).
I see- I get the idea, having him serve as a reason to keep on moving forward with the girls, but I would highly recommend to be cautious with how quickly he's able to do this.
NTR is always going to be very divisive, and a lot of people are likely to feel quite frustrated if they trying to get a girl, only for him to suddenly beat them. I'd recommend making his progress rather slow, and perhaps giving the player some sort of heads up so they can see who he plans on going after so they can 'counter' him. So he'd still be a problem if you aren't paying attention and ignore his next target, but people who don't want him to steal anyone away can do so without much stress.