I just received a reply from Yamako (Kinuyo Yamashita), but unfortunately her English is not so good, and she didn't know who the creator was or, indeed, who A. Nagata was.
> I have a quick question on Demon Castle Dracula/Castlevania. Please, do you
> know, who was creator of the series, maybe the person who said, "let's do
> Dracula game"? Was it A. Nagata? Did he also produce/manage the Famicom (FC)
> team?)
Maybe somebody with some Japanese language skills would fair better.