The cat on the right is one of the several my mother took in for a while, then found a home for. (She was sort of the neighborhood humane society.)
Lucy was also a stray...until I took her. My mother found her living out back, munching squirrels.
I've known my cat (11 years) longer than I've known my wife (seven years.)
Because Lucy lived outside for a while, she has mean streets approach to life that works likes this: humans are good. They provide food, affection, comfy places to sleep. Other cats are bad. They are competition and must be roughed up.
This approach doesn't work well now that we have two other cats, so Lucy deals with it by pretending the newcomers don't exist. Sometimes she HAS to attack them when they get close, which forces her to acknowledge them.
You can see a little of that in the picture above. Lucy is saying, "Get this other cat the hell away from me, or I'm gonna lose it." The ears are back...