As for the Vita, Kate is definitely the only rational choice. Not to mention that her gravity powers will add something new to roster.
This too. And i definitely agree with the cross-marketing part.
Kat will have a lot more relevance by the time Battle Royale is out, with Gravity Rush having been released months earlier, most likely to unaminous praise. Not only that, she'll be a much needed female addition to the roster and her inclusion will enable more players to discover her game and maybe even take more notice of the Vita as a result. Her role will definitely serve a purpose.
Plus she has a load of cool anti-gravity moves that instantly make her unique from everyone else.