She is approximately two years old. I have tried having a separate cage for the other rats that I have introduced and have put it right next to her cage. I have also tried introducing her to other rats in my bedroom with the door closed while I sit on the floor and she does not like any other rats at all. I am rather nervous about the aspect of even introducing a neutered male to her. She pins other rats down and bites very hard, hard enough to leave open wounds. I am working on the immersion training with her now and she seems to be doing really well with it. She is a total sweetheart with me but no one else. When my boyfriend approaches her she just runs away and comes to me.