I think you're unnecessarily hating.
They have not even kissed yet. Riena owes him her life and she's actually really helpful so that's why she acts the way she is.
They literally have no concept of privacy like that with the hot springs as it was pointed out in the manga if you had carefullly raad it. Riena did not know he went down because of "privacy" and he didn't even come down because of privacy (he went down because he did not want to lust over Riena, essentially). Riena thought he went down because it was too noisy and as she really likes him she went to check on him and spend time with him.
You can't blame them for liking each other. They're also both the same age and they're 15. You know kids are having relationships at that age like, what? It's not unrealistic that they like each other. Besides they still have not even kissed yet. Also, based on the passage of time, theyre probably close to being 16 too.
Honestly Reina kinda pisses me off. I can't put my finger on it but I think its because she's too pushy. And Hiel needs to be more stern instead of just letting himself be pushed around.
I cant bring myself to like Reina because she has no concept if personal space and just wont leave MC alone. He obviously went to the underground hotsprong because he wanted privacy and she decided to follow him regardless of that. If a man did that he would be called a pervert, so why is it okay when she does it?
Not to mention that she's literally a child. I'm not sure how old Hiel is but when Reina first showed up, she looked like a child.
This has way too much fan service. I'm so tired of female characters being written this way. Idk tho maybe I'm just looking into it too much. It's just icky to me tho that there are scenes where children like Reina and Rulle are shown without clothing.