Roxas charged. Angry opponents were predictable opponents. Vanitas had been on both sides of that equation, and knew that rule well.
He blocked a slash from above from Oathkeeper, only to step back when Oblivion swung up from below. Dualhanded Keyblade Wielders were rare. This would he exciting.
Was it a condition of that face to fight like no one else?
Was it a condition of the face to challange Vanitas like no one else?
Was it a condition of the face to enrage Vanitas like no one else?
He needed to be quick. Quick and deadly. One thing to set his opponent apart from the other was his strength. No doubt, if Vanitas got caught between those two blades, he'd be done for. How exciting.
In return, Vanitas favoured speed. Combine that with Sora's miraculous versatility, and you've got a deadly combo. While Roxas was no slouch on his feet, Vanitas favoured this style for years.
Gradually, he felt the magic build beneath his skin. Each blow, parried or taken from eithet side, intensified the feeling. This was the challange no Heartless could give him.
Vanitas jumps up high high high, bounced off the last remaining tree and dove at Roxas. Even with his focus on sheer power, Roxas leapt out the way in the nick of time, forcing Vanitas to turn his landing into a roll instead.
He jumped back up to his feet. Roxas didn't waste a second waiting and lunged again together with a burst of light.
This time, when Vanitas caught the first attack in a lock and launched a Firaga with his free hand, something changed.
The magic building up exploded into shattering glass, englufing him. The one magic of Sora's he'd never tasted.
Form change.
Black and white and blinding light. Light so very cold, even Vanitas might wield its borrowed potential. Shimmering, iridescent blades replaced the weapon in his hand.
Roxas retreated back to where the girl stood in the distance. She almost dropped the strange box in her grip, and gazed upon the enemy.
Floating above assorted dust, Vanitas tasted pure power.
"Let's see who wins now," he said to the face he loathed so dearly.
Ultimate Form.