She is aggressive and he doesn't care!!! Is she going to accept him one day?

So this is about the cat Petro, the male I rescued 3 months ago from a petrol station. I went through a lot with him until he recovered; he was a shy very sick scared little cat who didn’t leave his box for weeks (except for food and to litter box). I’ve put him for adoption here on EW but since my fiancé would like us to keep him, I thought maybe I can try to introduce him to my female cats, who in the past, did not accept other cats at all, both males and females.

Petro was so scared at the beginning to leave his toilet, where he was staying for the past 3 months. I was locking my cats in my bedroom and let Petro discover the house, but every time I let him out, he starts shivering and wants to run back to his toilet. This continued for several days until one day, I let my Siamese female “Cookie” out to see him, and immediately they connected. He doesn’t hiss anyway but even Cookie didn’t hiss at him and they started playing together. They actually play a lot Hide and Seek, they don’t hurt each other or bite or scratch, just very soft fun play.

The problem is with my Persian “Princess”, she is friends with Cookie, they don’t sleep next to each other but they play nicely together and they don’t fight at all, though Cookie, the Siamese is the dominant, and sometimes she hits Princess if she finds her sleeping on her tree and Princess doesn’t fight back.

Princess follows Petro and hisses on his face, he stands and looks on the other side, as if she is not hissing at him. If he passes near her, she jumps on him, scratches him and bites him and poor Petro continue walking as if she didn’t do anything to him. I haven’t seen any cat in my whole life careless as much as him. He is just so happy to walk around the house, to discover things and to play with Cookie.

The look on his face says “I am so happy in the house” but the problem is with my annoying Princess who is scratching him all the time.. I am afraid I have to keep searching for a good home for him if Princess keeps acting aggressive with him because he really, deserves a better than this.

Do you think she might change her attitude with him? I am locking him in his toilet when I am outside the house, but he is really happy when he comes out.