Our two cats have never been the best of friends. The long haired tabby tomcat is constant hassle mode to prove his alpha cat status, while the siamese female just sort of tolerates or even ignores his efforts. I think they respect each other but they remind me of typical human siblings who squable periodically as part of their life in a shared household.
The Siamese had to go in for surgery to remove a growth on her neck. When we brought her home, she was groggy and sort of like a lump that just wanted to lay in the sun. The tomcat suddenly became her comforter and protecter. He did not leave her side for 3 days. Wherever she was, he was not more than a few feet away, just laying there and making sure she was ok.