I met a wonderful woman who I wished to become my wife. When she moved in there was a clash of wills so strong the world has not seen its like since the Cuban Missile Crisis. She wanted a far greater share of the blanket and the bed than Duncan was willing to give. Duncan Idaho went from assuming this was another human to rule into deciding she was another cat trying to take over his territory. In short he went to war.
Any good general knows the best tactic is to divide and conquer.
When it was only Michelle around he would randomly hiss at her, maliciously destroy her possessions, and try to intimidate her in every way. Michelle is a remarkable woman. She staked out certain areas that were for people and not for cats. It also included the bathroom, particularly when she was using it. Both cats had learned early on that a person sitting on the toilet had nothing better to do with their hands than to pet kittys.