what would his/her specific line be like

"I pity the fool who don't use the powers of darkness!"

He would say nothing and call in the "A" Team.

He probably wouldn't actually use his keyblade, he would just wear it as a necklace. He wouldn't be aligned with any side, dark nor light, because it doesn't matter to him. And above all, "I pity the foo' who tries to make sense of this!"

Barret from FF7 if he makes an appearance.

He should've voices Lexaeus

"I pity the fool who don't use the powers of darkness!"

THAT WOULD'VE BEEN SO EPIC