That sounds more like a twist ending, the big reveal with "haha! I was acting this entire time!"
Having the protagonist act one way the entire time and then pulling the rug out from the audience will leave them feeling a little bit confused and annoyed. If they've rooted for a genuine person all along and then they turn out to be a bitch, they won't feel satisfied.
As to your question: in order to do a delayed character development, you really need strong supporting roles that the audience can identify with. They are the ones that can be developed instead of your protagonist, and the audience can follow their progress instead through the eyes of your protagonist. That can then affect your lead role later in the story.
If there is someone that your protagonist has worked with all through the crisis, and then at the last minute steps on to them to get to the top, the audience can at least sympathize with the support character.
Think about The Sixth Sense. The character played by Bruce Willis is the protagonist, yet you follow the story of the little boy through his eyes. You understand very little about anything of Bruce Willis' character until the end. The mystery about him allows the shock reveal at the end, whilst still following the story of the boy. If the kid wasn't as primary a role, either the reveal wouldn't work, or the film would be about nothing.
The only way you can have a character change, even if it is the main character, is to reveal nothing about them. However, you can always show who that person is behind closed doors, and allow her to reveal herself to other characters rather than the audience. It's OK to have an unlikable protagonist.
Think about Carrie from Sex and the City. She is intensely unlikable, and just a horrible person, yet the story is told through her eyes. But she has a very strong supporting cast that make up for her failings that the audience can identify with.
My advice would to put a secondary character in a more prominent role. They don't need to be the main character, but an audience needs at least someone to engage with throughout the story.