Takeuchi: Speaking of being powerful... Who's stronger: Ciel or Kirei?

Nasu: Ciel is by far more powerful. Just take her lineage into consideration, and add to that her immortality. Kirei's prowess in "Zero" was rooted in the sheer number of Command Spells at his disposal as much as it was in his obsession with Kiritsugu. Suffice to say, it was his golden age. I believe the "Zero" Kirei could have defeated Ciel.

Fate/complete material II; Characters: Kirei Kotomine, pg. 66-69