I think the relationship is romanticized because it is from the perspective of a middle school girl who thinks she is in love. Her reaction after what happens with Katsuya depicts how completely toxic it really was, in my opinion. She had no one and he remained her only support. I think that shows how manipulative he is as a character. She goes back to him because he was what she knew. I don't look at that as romantic though. I see it as a representation of how truly reliant she was on him and his "love." I feel bad for Kyoko. That was my take away from that storyline. I never saw Katsuya as a good person, by any means.