Ariel
Hate to jump on board, but I have to agree as Squirrelflight being my least favorite character.
I actually don't mind her now, but when she was an apprentice and a recently made warrior… nope. She is one of the main reasons why I refuse to read the second series ever again. I just can't stand her. One of the very few characters out of all the hundreds of book I've read that I can't stand.
Her personality just gets to me. The way she snaps at other cats when she doesn't get her way. What comes off as her bullying or just bulldozing over others to get a chance to speak. I just.. ugh. She always just came off as a spoiled brat. I wish the Erins had some sort of excuse for her behavior and had her have a realization to stop being such a jerk all the times, but nope. I don't mind that she stands up for herself and doesn't like others defending her, but the way she goes about it is too much for me.
I wish I could go into better detail or give some examples of what exactly bothers me, but there's no way I could bring myself to go through those books again.