I don't actually think Riri means "devilish" as an insult. I mean, Riri is literally a devil lol.
Keep in mind, Luna has been told her whole life that she's cold and unfeeling, like a doll or something. Riri is saying the opposite: that Luna is highly emotional and passionate, that her strong desires are precisely why she wanted to stay friends with Mai post-breakup instead of just cutting her off. Of course it is true Luna was taking advantage of Mai, I think Riri means that, but I also think this is Riri's way of trying to build a connection with Luna: "you and I are similar, we're both selfish and put our own needs and desires first, and that's okay."