Last week I finished reading Lauren Oliver's BEFORE I FALL. It was on the long side, but for the most part I thoroughly enjoyed the story from start to finish. The protagonist, Sam, isn't a particularly sympathetic character. She's a bully and her friends pick on those weaker than them. Although I found some of her actions abhorrent, the connection she had with her three best friends was very genuine and heartfelt.
As is typical with YA books, Sam undergoes an emotional transformation over the course of the story. She begins to feel awful about the way she's treated other people, and she starts paying more attention to her childhood friend, Kent, who has always had a bit of a crush on her. I think it's important that characters go through such a transformation. However, they must do so without pulling a complete 180 and acting in opposition to previous character development.
This was the one thing I loved about Sam. She realizes the error in her ways and becomes a more empathetic person, but she never ditches her friends. Popular kids are often stereotyped in books. Here's a typical scenario: girl social climbs, starts hanging out with popular girls, becomes really mean, has an epiphany, realizes being popular isn't important, and ditches her mean popular friends to hang out with the nice nerdy people.
To me, this isn't realistic. Popular people are capable of loving one another (even if they're sometimes mean to others). Even though Sam changes and recognizes that her friends aren't necessarily the nicest people, she still cares deeply about them. This led to a very satisfying transformation. Rather than coming to the conclusion that "popularity doesn't matter and thus popular kids are awful," Sam acknowledges her friends' faults, yet still loves them deeply, just as much as she did before. Because people aren't perfect. Situations are never black and white.
So when it comes to your character's emotional arc, always keep in mind their underlying personality. It's unrealistic to have a protagonist suddenly change all their values. Sam changes to some extent, ultimately sacrificing a great deal to help the people she previously tormented, but she's still the same person, who loves hanging out with her friends and dressing up and going to parties.
BEFORE I FALL is a great example of gradual character development. If you haven't already, I suggest you check it out.