Easiest answer : One was honest with their intentions, the other was living an entire life of a lie because she felt necessary to do it for the sake of "Duty."
More complex answer : You'd have to be in a situation where a girl, a friend, or someone close to you in some medium openly reveals that they never really liked you and only stuck around you because of a reason that was personally beneficial to them. Thats why we dont like Kaija.
I also dont think people are really open to the elves, themselves, but open to Medb and Finn moreso. Even from my viewpoint the Elves are also corrupt, similar to the templars theres racism on both sides that controls them and has bad actors in both parties. Even Medb seemed to notice that. However when it comes to Finn, Finn was openly against you, hated you deeply, wanted to kill you in every way, until she was put in her place, then she came around, she shared genuine authentic moments with MC and had a deep connection with him, even going as far to change her entire worldview of just being a racist piece of shit cocky elf to an understanding, compassionate level that unfortunately ended up in her own demise standing between the Templars and the Elves, some slight foreshadowing for MC if he were to try and do the same I guess. Basically saying he HAS to pick a side if the time comes.
Kaija's entire life with the MC was a lie, her entire romance with the MC was a lie, theres very small choices you can pick, that can affect the relationship enough to see how Kaija probably would genuinely feel in that situation which honestly makes the most sense because you cant genuinely force someone to love you just because they spend all their time around you. A lot of us probably see that for what it is. The fact that if you just go down a very slight deviation with Kaija she basically goes completely cold towards you, then the alternatives is a "we should be apart but still see each other" or "straight love" route. Kaija is very cold in every route outside the flat love route. She doesn't treat MC like a lifelong friend in a lot of ways, she's very upholding to her duty as a Templar, even when theres blatant problems she tends to feign ignorance, she's also not really there for the MC in the ways you would expect someone to be that's been with you, your entire life.
Thats how I view it anyway. It's hard to trust or respect a person that was set up in your life as a plant and just doesn't try any harder than they should to stay with the person. The whole entire relationship is built on a house of cards. If you're cool with that by all means, but all in all, I would never be able to be with a person like that lol. If I found out 10 years from now my wife was a plant by my parents because they didn't want me to marry some harlot that was going to destroy me and wanted to give me the illusion of a stable relationship, I'd likely kms. Would be such an insanely huge betrayal by not only my family but my own wife lol.