Rua probably is a M in disguise, MC should just correct her for her behavior, she will love it
I don't really hate her much, Rua is a tsundere who is harsh with her words but there is also blame in MC for being too passive and just accepted everything a tsundere said. They are childhood friends, it is a bit dumb of him that he didn't realize anything about Rua, it is not like she try hard to hide her feeling at all.
I actually like Rua more than Amane, Rua seem more interesting, she will probably fold with just a kiss or a hug from MC ( MC is just too dumb to not realize it, he deserved to be treated like that 100% ).