Not much of a ruka fan here but i dont hate her though. If I remember correctly, she has an illness of slow heartbeat right? and was afraid and tried to be a "rental girlfriend" to find someone who can make her heart beat faster. Thats what I understood from that, I might be wrong. But we can also say that she's manipulating or the one annoying kazuya. She might not be in love, BUT the girl just wants to fall in love and wants it with kazuya. Yes she's still young and might not be mature about their situation and who can blame her for that? If anything, kazuya should have the spine to reject her if hes In love with chizuru and not drag this any further.