whoa, all the confusing rage.
Seems to me that Remi's being presented as a person with good intentions and a vile nature, so she's being beneficent by being awful. She wants to disrupt the passivity that's ruining Sacchan and Ruki's relationship. Sacchan will go along with anything because the alternative for her is blind, unhelpful anger, and Ruki never believed that she had anything resembling a chance, and she's wanting to put off the rejection as long as she can.
I admit, I don't understand the apparently common belief that cheating in a relationship demonstrates a reprehensible, rotten, and unfixable moral core. I mean, I wouldn't like it if it happened to me, but I also figure that it's something that takes place in the context of a relationship, and so it's up to the partner to forgive or not. Regardless, a lot of character nuance seems to be glossed over by people who are just mad about the cheating.
Oh, and so Ruki's officially envy. That makes Remi.... greed? Makes sense.
I'm hesitant to form any lasting opinions yet, because there's still two chapters to go, but I can sort of understand what motivated Sacchan's behaviour - that doesn't mean it wasn't incredibly shitty though. I don't think one indiscretion means the end for a relationship, particularly if they're willing to discuss it, but she apparently fucked Remi behind Ruki's back for an entire month, and in general has just been a shitty fucking girlfriend. But I don't think that means she's an awful person. Most people's first relationships are training wheels relationships, where they figure out how to be with someone and deal with doing shitty things to someone you love and having them done to you. But I think in this regard they've gone past the point of no return and should each cut their losses.
But I know they won't, clearly, because Ruki is stupid over Sacchan.
Also, /u/ anon messed up first, go check the thread again. Ruki's apparently pride, Remi's envy.