Xion might be panicking. First, seeing a red-eyed Sora might have set off some lingering identity crisis related issues Then, because that Sora and Roxas made a classic example of hate at first sight. Then, because they actually started going at it, and lastly, because – despite Riku briefing everyone on the situation via messy GummiPhone call – Xion was decidedly not prepared to deal with Ultimate Form.
She'd seen it in action, once. And then she vowed to never make an enemy of Sora, which was impossible anyways.
The first course of action she took when Vanitas – she knew it was him since she paid attention to important briefings - showed up was cram out her GummiPhone out of one of her many non-coat pockets. After that, while Roxas did a great job of either deliberately or accidentally keeping Vanitas distracted, she dialed Riku, saying something or other that amounted to "Hey, Riku. Vanitas is here please come over quick?"
When she started hearing metal on metal, she realized that maybe, she should've known emotional talks with Roxas had a history of turning out this way.
Her GummiPhone barely remained in her grip as she couldn't help but flinch. No one could avoid it, when their adversary, floating a meter of the ground and surrounded by a grand total of nine cosmic blades flashed into existance right in front of their nose.
Instantaineously. Like he'd simply taken a step ten meters wide. Next, Vanitas swung his blades with a mere flick of the wrist. Roxas, who'd briefly turned his back to retreat, would no doubt be struck down after Vanitas' step through space. But instead of meeting such a grisly fate, a key – a Kingdom Key?, Vanitas thought – flashed into existence in time for each sword to bounce off one by one.
Vanitas' memories of time spent with Sora swam together as a dream. He knew names and people and places and events, yet even still, just in the vague way of a particularly striking dream, not that he had much experience with those, considering that before, he'd never dreamt at all. Even among the runny impressions of Sora's friends, Xion reigned as the vaguest without a doubt.
No matter how much he tried to think of her, most of it seemed to… skip right over her. Maybe, Vanitas wondered, this was why he hadn't noticed her before.
Maybe he should focus, now, on the second person to enter the fray, no matter how much his eyes strain to look.
And maybe, Xion thought, she could buy enough time for reinforcements to arrive. Now was her time to be strong.