[Ithekro]

Well there is one other possibility, a holdover from the Third War....but there would have had to be a war for it to happen.

Spoiler for Third War:

Avenger.
Though he's suppose to be in the Grail or something after the war because he's techically "evil". Though it would make sense with the "evil" cards they gathered...or the "evil" spirits that the cards summoned such as Rider and Caster. And perhaps why the heroic spirits are all corrupted versions so far. Though Gil doesn't look corrupted....just female.