She's the most irredeemablely disgusting person in any manga I've ever read (except berserk), she has no motive for the abuse, no real goal or purpose. Even if the next chapter shows her being completely head over heels in love for him in a flash back or something, no one would support or feel any sympathy for her. Her whole existence and introduction as a character in the story is pointless. I think the only reason she was ever in this manga is for it to get a bit of a extension to the story but in the process it's made all interactions with her in completely and utterly unbearable.
Thanks for attending my ted talk.