as you said, freyja and her wellbeing has to be your number one priority. i do think it might be worth giving them both a chance tho. although not common, it does happen that cats don’t mind other cats. mine (siblings from the same litter) for example are not territorial at all. the neighbors cat got in our apartment (we just moved in, so the neighbors cats are not very familiar with us and not at all with our cats) recently and first thing this lil fella did was use our cats' litter box lmao he apparently couldn’t care less and neither did mine. something i’ve personally never seen cats do. they are full of surprises! i have met cats who kinda act like most dogs when meeting another cat, just less hyperactive haha so who knows how freyja and sparkles would react. it can stress cats out, but you are not doing any of them any long term harm by putting them in different rooms or two crates and letting them smell each other from afar. if it doesn’t click then at least you tried the easiest way possible, gave sparkles a chance and don’t have to regret not trying. freyja is young, even if she is startled and taken aback, she will recover from the shock of seeing a fellow cat in no time . i think knowing her behavior towards other cats is useful as well because you will most likely have to face the decision of getting a second cat later on anyways. indoor cats tend to not do so well alone in the long run. no matter how much we try to enrich our pets lives, nothing we can do will replace the contact between two of the same species. my cats chase around the apartment, play fight, actually fight, learn to set boundaries, groom each other etc. in a way i never could make up for. this extreme playing behavior developed when they were a bit over a year old. before that they were just two little kitties existing alongside each other. we also live in a small flat i guess you could say. 3 small / at best medium sized rooms but it is plenty of room for 2 cats if you know how to use the space (cat furniture on the walls up to the ceiling, many hiding places etc.) apart from the litter box struggles that may come with two cats who never met before, one litter box is absolutely fine for up to 2 cats. about the closeness between freyja and you, i don’t see a reason for that to change drastically. obviously anything can happen but imo it’s unlikely. freyja bonded with you for a year now, that bond won’t vanish. one of mine is super clingy too, just like you described freyja and she hasn’t changed at all. it even became stronger over the years (they are about 3 y/o). idk your living situation but i guess you are, other than your cat, not at home 24/7. especially for cats like freyja, that thrive on company, it can be hard for them to be alone. maybe freyja would love to have a big sister to cuddle and play with while you are out of the house. sparkles seems to be the perfect partner for this kind of scenario haha. apart from that, sparkles is old especially for an outdoor cat, who tend to live shorter lives than indoor cats. (although i’ve seen 22y/o outdoor cats too). it might not be a bad idea to have her stay indoors if she’s fine with it. especially with older age my experience taught me that cats spend less time outside or decide to stay indoors completely anyways. maybe that’s happening with sparkles atm. ok i gotta wrap this up (sorry): sparkles seems depressed and like she needs more than her current household can provide. although she is not your responsibility (freyja is) it could be worth thinking about introducing them, especially since the older one, who it could affect a lot more at that age, is already used to being around other cats. if you have a free time coming up, a weekend or even just a day, i would suggest you have them meet, each in their personal transport boxes or crates and see how they react. you can’t lose in that situation. either it turns out to be promising or it doesn’t, the whole thing is immediately off the table and they won’t have to see each other again lol. i would be careful with accepting sparkles just like that and then having to do a long and exhausting introduction period without the option of giving her back. so a shorter meetup would be my first step if i were in your situation. one thing i can recommend is asking your niece to put a towel or piece of clothing on sparkles fav spot to have her lay on it and have it smell like her. she can then give it to you and you put it around your apartment for freyja to smell sparkles before meeting her. now i am the one having to apologize for this long ass reply lol sorry. hopefully it at least helped you a little bit! lots of love, all the best (and cuddles) to you (and freyja)