It doesn't surprise me that people don't like some characters in this film. What DOES surprise me is that the character that most of those people DO like is Knives? Really? I liked the movie, but I found her shrill and annoying. If there was one character I truly hated in the movie, it was her. It just baffles me that people liked her for any other reason than pity.
As one of the folks who liked Knives, I will say I agree that she's annoying and immature at first. But then she gets over it. As opposed to Scott, who supposedly "learns the power of something or other" even though I never saw how or why it happened, I actually understood what Knives wanted, and what she was doing to get it. Maybe I wouldn't want to hang out with her in real life, but I was interested enough to watch her from a distance.
And unlike Scott and BlueHair, Knives earned her victory. She had a goal, she tried different things to get it, and in the end was a different person for it. Which makes her the only major character with anything resembling a character arc.
Oddly enough the changed ending gave Knives an even stronger arc than the "real" ending did - when Scott ended up with the other chick, Knives was okay with it, and I was happy for her. Good for you, now ditch those posers and find a real boyfriend!