"Yuno Gasai" of "Mirai Nikki"
*She's like their school's most idolized girl, & known as a model student by everyone... But don't think this girl's just all that beauty, brains & character, like the girl of your dreams...
Because any girl who becomes too related or anyone who becomes a danger to that one man she loves, Yukiteru, she won't think twice of killing them with anything she can get her hands on, at that very moment, from knives, to injections, to cutters, to axes, or even baseball bats without a single remorse, & with a flashing smile... Bwahahahaha! How's that for Kickass!
Well, I'm surprised that she hasn't been mentioned yet, but she's one of the most badass girls I've ever seen, and she can rock a bear suit like it's nobody's business! And, she's from my favorite anime series of alll time!
Everyone has Saber from Fate/Stay Night but what about Rin Tohsaka? She was the one with all the plans, jumped in BERSERKER'S arms just to blast him in the face, beat the crap out of Caster, also has a plus +1 on the kickass scale for being the master of Archer.
She is also a witty sarcastic bitch - which I think is cool.
Oh and tifa lockheart too - see: Dead Fantasy and FFVII
Saber (Fate/Stay Night) -- I shouldn't even have to explain why she belongs here.
Inner Moka (Rosario+Vampire) -- From ogres to werewolves, she kicks the ass of every monster in existence.
Kallen (Code Geass) -- She was easily the best pilot of a Knightmareframe to exist. She kicked the ass of the Knights of the Round and even Suzaku.
Gasai Yuno (Mirai Nikki) -- She knows how to use a weapon to kill people. This Yandere has saved Amano Yukitero's pitiful ass many times in seemingly hopeless situations.