Regarding concepts, I was thinking simply when it comes to powers and such. A former knight (largely inspired by characters like Garland, as that's the closest FF character I can think of atm) who failed to defend his world from the Heartless and watched it fall, becoming encased in his armor as a result, which has itself been twisted by the Darkness. An unending fuel for his unending hate towards it perhaps, although apparently Nobodies don't feel emotions due to their lack of a heart. I'm thinking he can control black flames ala a sort of corrupted version of Firaga, and can freeze enemies in place via Stop. Beyond that maybe he could have a barrier ability or--failing that--an ability like Terror.