I can understand why people get frustrated with Ruka's character but what I feel a lot of people don't realize is that she's helping him understand his feelings more for Chizuru. Ruka hypes him up and even supports his efforts with helping Chizuru cope with her loss, so it's sad to see her relationship with Kazuya fall through. The story is between Kazuya and Chizuru after all, so it's inevitable.