Roxas flinched at the sight of the creatures attacking the locals, but not truly being able to feel anything for them let him be more detached from it. Somehow he didn't think he could afford the distraction of trying to help them. At least if Vanitas's attention was on him...

"It's Roxas," he said, summoning Oblivion as the helmeted boy approached. "I'm no fake."

But Vanitas certainly didn't seem interested in conversation, and almost immediately he was dodging out of the way of that fire. He sensed the portal opening behind him and whirled to meet the incoming attack with his own keyblade. But he was still just a little unprepared for the fight, and the move was wholly defensive.