Like with all of the characters in Frozen, his character development is horribly done. He starts off as a cynical jerk that hated people and treated Anna like dirt and all of a sudden he became sweet and sincere guy that loves Anna? WHAT THE HELL? He's so horribly developed that his back story is a plot hole!

As much as I hate him, I still have a good grasp on his personality.

Either way he still changed

He does have a personality and does grow to care for Anna. His development is rushed, but at least there's something going on.

Yeah, even if you think it's horrible development,it's still development, so he's not flat. He does change a bit throughout the film- more like come out of his shell.

I feel like all the middle arcs are missing in the Frozen's cast development

You could easily rewrite the movie and drop him out of it.

A rushed development is still development, he seems more open at the end of the movie.