My cat's best friend is one of my sisters-in-law's cats. She will welcome his brothers into our room, be friends, whatever. The one she pretty much helped raise, though? I'm sure if we asked them, he'd say she's his cat and she'd say he's her cat.

Sister-in-law gave her cats a bag of new little felt balls for Christmas. The best friend cat carried one in his mouth to give it to my cat. She now ignores her other toys, rolls around with the ball, and joyfully chirps!