Why hasn't he introduced Donquioxite yet then? He's an obvious addition, a beloved character of Western literature for hundreds of years.
He's actually in the form of a noble phantasm by Miguel de Cervantes as mentioned here. This is a comedic spin-off so chances of him actually appearing is low, but there are some here that made it so maybe one day.
Here I am remembering how guns stopped the development of noble phantasms and entry into the Throne of Heroes.
He also said 100% fictional characters, as opposed to mythological, also would not qualify. And here we are with Frankenstein's Monster, Sherlock Holmes, Jekyll & Hyde, and goddamn Nursery Rhyme.
He also said Manfred von Richthofen qualifies as an Eirei, despite like Billy here, there being nothing remotely "miraculous" about his life or feats.
Then pure scientists like Babbage, Edison and Archimedes also qualify as spellcasters for who the hell even know reasons.
Nasuverse rule changes however the hell it's current writer wants. Consistency and logic is thrown out the window because they think "convoluted wall of texts" equates depth and sophistry as long as it's "hard to understand".