Part of the Sumi problem here is that she's essentially a public figure that lives off of popularity, so if she continues to be a public figure it's going to be really hard to separate the massive benefits she just got by conveniently announcing her relationship right at the moment when the nation is calling him a hero. Essentially latching onto his rise in stardom.
Another part of the problem is the fact that she kicked him to the curb, then suddenly decides to pretend it never happened and announcing that to the world, while he can't object (he's unconscious). Does she know for sure that he hasn't moved on to someone else? Does she have any idea how he took her rejection of him? Does he even still love her? She doesn't care, she one-sidedly decides to declare to the world in a hyper public way at the height of HIS popularity, that she's his girlfriend. Even if there is no dark side to this, it's going to hard to avoid the implications hanging over them.
Of course for myself and many others, the biggest problem overall is that Sumi just brings nothing to the story here. She's a bland hollow character who's just been on the receiving side of the relationship with the MC. When she has a chance to express personality or real emotional expressions, she doesn't, or she does the opposite and dumps the MC. She's a terrible choice unless the author is going for a partner who won't overshadow the MC in any way and simply fill the role. Like others have said, no one really hates her, she's just not a good character and is a negative influence on the reading experience because of it. IMHO anyway.