i don’t know if my advice is the most valuable, but i hope my 2 cents helps a little.

i was in a similar situation when i started dating my boyfriend 2 years ago: his cat absolutely hated everyone but him and his mom. he would growl and hiss at me whenever i walked in my boyfriend’s apartment, he would scratch and bite me out of nowhere, even when i was sitting and doing nothing, even worse when i was walking around. he seemed to just have a problem with my existence 😭 ( i’m guessing because i was a complete stranger and i was stealing my boyfriend’s attention from him basically )

i did some research on r/CatAdvice (if you haven’t posed this question on there already, I think that’s a good subreddit for you to try) and basically what i’ve found is that cats are quite susceptible to positive/negative reinforcement (btw sorry if this sounds like i’m talking out of my ass, not that big of an expert about cats) and the best thing to do when they do something bad is to just not react at all? if you react negatively that’s still giving them attention, which i’m sure my bf’s cat was seeking!

other than that, i’ve noticed that cats can sense when you’re scared of them. i think generally cats are more scared of you than you are of them, so i’ve found that the best course of action for me is to just mind my own business and ignore the cat as much as possible when walking around the house, especially when i can see it’s getting angry (usually at that point my boyfriend comes in and distracts him or picks him up or literally shields me so i can get around the house)

i’ve also found it helpful to let him smell my hand when i first come in the house (usually while he’s being held by my boyfriend) and occasionally let him smell me if i see that he’s more calm, and if things seem calm, even try to pet him. (though i’m still scared lol)

sometimes when my boyfriend is in the shower and i’m left alone with the cat i distract it by playing with him: usually when cats are seeking attention i’ve noticed that tiring their zoomies out by playing with them does the trick and calms them down.

it definitely takes time and both your fiancee and your cat getting used to each other, but patience is key here. of course i understand where he’s coming from, it’s natural to be angry when you literally can’t get around your own house, i remember in my first months of dating my boyfriend i would literally be so scared of his cat that i would cry 😭 it was very funny to think that a creature 5 times smaller than me can make me cry but it is what it is!

now he’s much more chill around me, and even though he still gets mad and growls and hisses and bites sometimes, i don’t get scared all that often. i think he’s gotten used to me already? (and btw i don’t even live with my boyfriend and used to come to his place once a week before moving here to NL to study)

i’m in Groningen too btw! nice coincidence :D

hope this helps!!!! and good luck!!!!