Yup is jealousy alright.. the plot won’t be that complex to be something else.. if it turns out to be something else, then I’m happy to be wrong..
I really hope it's something really deep like her being a knight that is against the whole saint system thing and thinks royals like the princess are incompetent to deal with the duty the magicians and knights do on a daily basis. But all of it is because she doesn't know better — that a very powerful magic only works with a summoned saint and someone with royal blood and is not something that can be replaced.
Idk, I want her to be a closet anarchist or whatever, but against the saints and the fact that they aren't even from this world to be concerned with their issues.
But yeah, most likely it'll be a long-term crush on the princess she had and now that she's seeing this girl just kissing her highness left and right it's pissing her off.