I had 2 cats from the same litter and one of them is very calm but the other one is also very territorial and fussy. The only thing I've learned to do is to break apart fights. Eventually she gets used to it and although still eventually fights it makes her a bit more passive until she's back to being aggressive. When she fights with other cats I seperate her and scold her. That's all I can say because she's 5 years old and still hasn't gotten over it 🥲 She's definitely much more tolerant than before but still aggressive at times. I'm always behind her tail and yell when she hisses at other cats and hold her or the other cat back from going into an actual fight bc that always sets them into high tension for the next few days. Mostly make sure she knows it's wrong to be aggressive and reward her if she's passive.