Who is Bull MacGregor from Mrs. Mike the Story of Katherine Mary Flannigan and what is their importance?

As the former business partner of George Bailey, Bull MacGregor steals George's woman. Bull represents physical strength. He beats the Indian woman repeatedly and she thinks he will kill her if she leaves him. The Indian woman asks Mike to arrest Bull however, Mike knows that she will not file a complaint. She will stay with this man who treats her poorly. This is another source of confusion for Katherine.