This game should not be a popularity contest and the reason why you havent heard about Kat is because she is brand new as of 13th June 2012. Assassins Creed has been around much much longer, 2007. So obviously more people would know about that. Kat is a first party character so she immediately has better rights and a better case for inclusion over Ezio. Just because you dont know a character do not presume they are instantly **bleep** as you are doing. Do your research! and you will find she has amazing gravity abilities that could work well in the game.
Ok so gameplay wise of course we like the game where you get to run around killing people more then the artsy action adventure but charecter wise? sure ezio is the best protagonist so far but he is still just a womanizing murderer with decent humor. kat is a compelling charecter. one people root for with abilities that are quite creative.