Well I will be cursed for 1000 years...
...Listen guys, girls and anything inbetween. DmC is JUST a game, with a story. Wow, I never knew. And before a lot of you shoot off your rightful opinions, I think it's a nice time for me to mention something. Kat wouldn't be in the game if she wasn't in some way useful. The same applies for every character. Each character is useful to a degree given the nature and cause for them to be in the story. Gee, fair enough if you write a character that has the brain functionality (also a big word, may I be so bold to say) as a lemon. Say a man called Fred who does nothing but stand still...
...Kat, Dante, Vergil, Mundus, Lilith and Dillon who was an unmentioned guy who worked at the Virility factory. He uses sarcasm to make points
They were all useful given the ways in which they were portrayed through DmC's story. This is coming across as some war to find reasoning behind certain character's actions. That's called character development, building each character into and around a story and giving them their own personality, look, style, structure and goals. They're all useful to building up the story. Kat included.