Following you around and howling if she can't find you are definitely signs that Kitsune needs reassurance. I think you need to know why before you can know whether things will work out between her and Rinoa.
So it could be behavioral: My Celia was always a lap cat, but she needed even more reassurance and more attention after my other kitty, Margaux, passed away, even though they were never close in the 13 years they lived together. I was just thinking that also took her months before she stopped looking for Margaux every day, so it could be that Kitsune is just now accepting that Koneko isn't coming back and feeling depressed. Make sure to give her lots and lots of attention, and try one of the anti-anxiety remedies that I mentioned.
But it could still be medical: If they didn't take her blood pressure at the recent vet visit, I think I'd want that done. Margaux went completely blind when she was 9, due to undiagnosed high blood pressure. I missed the signs and later realized that she had become more clingy, even though she'd always been independent and didn't even like petting. And if vision loss due to high BP is caught early and treated, it can sometimes be reversed, or at least further loss can be slowed or prevented.
Do understand that I am completely paranoid about high BP because of my experience with Margaux, and Kitsune's BP may be absolutely fine. But my paranoia did save Celia's sight; when checked, she also had high BP. It's more common than you might think, especially in older cats. Kitsune isn't a senior yet, but a BP check isn't terribly expensive, and it's not invasive, so I would consider having it done.