New cat

[Mariska]

My neighbor is moving to an apartment on the tenth floor, and now she asked me to adopt here main coon. He’s a really sweet, cuddly and funny male cat, and he already lives in my house and garden most of the time.
But the problem is: he doesn’t get along very well with my female cat. He attacks her from time to time and she doesn’t want to have anything to do with him. (Most of the time they just ignore each other).
Now I just put him outside if he behaves like that, but I’m afraid to leave them alone in the house together during winter time.

Does anyone know what I can do to make them get along better?