For example, Old Mog likes to be petted – she shows this by laying next to you and staring at you. When you stroke her she melts into a puddle of contentment and you occasionally stop so she raises her head in outrage and you can confirm she’s not actually dead and you’re not actually petting a corpse. Because that would be creepy.
Now Sibling Kitty screams and begs for attention until you finally pet her at which point she has a near seizure of joy because ZOMG HUMAN TOUCHED HER AND IT’S AWESOME! All four limbs flailing as she twists around and probably does herself an injury. The worst part is she kneads with all four claws – the air, the sofa, cushions, fleshy bits of human – just flexing away little razor blades in every direction. She also demands attention because while the Old Mog will just puddle, Sibling Kitty will not only flail around with razors but every now and then will suddenly decide “KILL THE HUMAN!” and you need to pull your hand back quickly or be flensed to the bone. Doubtless this is to ensure human reflexes are kept at peak efficiency.
Old Mog is tolerating Sibling Kitty’s presence. She still hisses, but it’s more a “I would like to formally announce that your presence in this house is unwelcome and I disapprove most strongly” hiss, rather than a “I will kill you and wear your skin on weekends and high holidays” hiss.
However, she has developed a “you have betrayed all that is good and pure in the world, there is now a desolate wasteland where hope once resided and barren void where kindness once lay” glare every time she sees me or Beloved with Sibling Kitty. It’s a very eloquent glare.
Feeding time is… vexing. Both kitties will appear at exactly the same time and demand food. At which point both kitties are convinced that a) I have put the sublime ambrosia that the gods would kill for in the bowl of the other cat while simultaneously dumping goat excrement in their bowls; and b) that their bowl must be protected at all cost, under no account must the other cat come within a yard of their bowl or Death Must Follow.
This is more complicated by Old Mog needing medication for her super-scabby skin which now has to be hand given to her (oh dear gods cats and pills) because there's no guarantee who will eat the food
The most annoying habit of Sibling Kitty is her yowling. Now Old Mog yowls. Old Mog yowls a lot – but she’s adept at communicating what she wants with those yowls. If she yowls near her food bowl, she wants you to fill it (that doesn’t mean she’s hungry and she will, inevitably, walk away from said food. She just wants to test her human’s obedience). If she stands near a door and yowls she wants you to open it (again, not necessarily to go through it, she just wants it open). If she yowls near a chair she wants you to sit down and pet/comb her.
Sibling Kitty will sit in the middle of the floor and just yowl. She can keep it up for hours while you play a guessing game as to what this silly creature actually wants. I suspect she likes to hear her own voice.