My immediate thought seeing this was "demon summoner." Which quickly transitioned into looking up significant figures in demonology because I know jack all about demons and their summoners. This lead me to
The Grimoire of Pope Honorius, which is a book instructing how to control demons in the name of God that was purportedly written by Pope Honorius III... but was actually made four hundred years after his death and is speculated to have been an attempt to trick churches into performing demonic rituals. And I'm reading this and am like, "Oh, this is
perfect."
So, I was thinking a Faker/Pretender (not 100% sure on the difference between the two) who claims to be Pope Honorius III as a Caster, but is actually the nameless author of Grimoire trying to bring the masses under his dark god's influence. Giratina can even stay as an avatar for said dark god, which is, of course, summoned as part of his NP.