Ada's isn't even available until the other three are completed.
I understand why. Just tried her campaign and got my ass kicked. Do you just not have a partner? Also want to point out my one and only gripe with this game having completed 3/4 of the campaigns; the missions are too long. I'm spending about 90 minutes per stage, if I'm playing with someone who knows their way around it's more like 50 minutes and while I love the campaign I don't always have a hour + to play a game. Check points are very reasonably placed, but as soon as you leave the game you lose your progress and it seems like breaking up the levels like they did in RE5 would of been better. Besides that, I can't see why this game got so much hate, wish I would of got this at launch
That isnt even the reason why. Her campaign is a spoiler for the other 3.