Probably a dumb suggestion but… I know you can get cat feeders that will detect your cat's microchip and only let them in. I wonder if you could get a litterbox that uses similar tech to only open for Kika?

When you find out, please please let me know. I have three cats, five litter boxes, and still everyday the one shits on the floor beside the box. I'm actually happy to clean up the poop because about every third day she also pees beside the box which is much worse.

I had a cat like this. The only solution was to scoop the litterbox multiple times a day, so it always seemed fresh.

There are also auto-cleaning boxes.

Try using a different brand of litter in Kika's box? If you're exceptionally lucky, you'll land on a litter that only Kika likes.

Try training Kika to use the toilet? My Grandmother's cat was the same way back in the 80's, and my uncle (yup, he still lived at home in his forties) trained the cat to use their downstairs toilet. Problem solved.

Edit: we kids always tried to catch her doing it but never did, but wed see the evidence after the fact.

I see you mentioned elsewhere that automatic boxes probably won't be sufficient. While I generally wouldn't recommend a litter box that fully disposes of the waste automatically (because scooping the poop might be the first or only warning sign of health issues), something like the Cat Genie might help.

If Kika is willing to use an enclosed litter box, you might create a litter box enclosure using one of those pet doors that can be set to unlock only for specific animals based on their microchip or a collar tag.

Thank you guys for all those ideas! Sorry for the late reply.

A few highlights on ideas mentioned here:

• @0x0@lemmy.dbzer0.com's idea of testing different litters might work. Frieda used to be a street cat, so she'd rather use dirt or grass (or finer litter), while Kika was always a home cat so she prefers more typical gravel-like litter.
• @atx_aquarian@lemmy.world mentioned odour removers. I didn't try them, I used alcohol instead. Got to try it too.
• A lot of people mentioned smart systems that allow/deny entry for a cat based on the microchip. I gave those a check, and they're outright expensive here in Brazil. (Import taxes are specially harsh on electronics, plus cost of living is smaller than in Europe and CA/US so stuff like 100 euros or dollars is actually a big deal here.) I might want to try a poor man's version of that though, by not allowing Siegfrieda to access my bathroom at all, and placing Kika's litterbox there.

A relevant detail that I didn't mention is Kika's age - she's already 16, and cats get a bit stubborn when old (not that we humans are any different...). But I think that a mix of the solutions that you presented might work.