I have never played Monotone Princess, nor have I tracked the storyline clearly, so I will be referring to the Wiki for this question.
The formal explanation is that Okubo had made the character design for Grimoire, then liked it so much that he added it to the manga in the Baba Yaga Castle arc. Since Grimorie already had the book motif, that meant adding the Book of Eibon to the story.
Otherwise Grimoire (who may be known as Noah as well in the video game) and Noah are as different as Anime!Maka and Manga!Maka, or Anime!Stein and Manga!Stein: some differences, but some close personalities and abilities. I wouldn’t say they are as different as, say, Anime!Justin and Manga!Justin, though.
I think it is debatable how long it took Okubo to make Noah into his own character, as the chapters had hinted he may have been Eibon himself—before we learn he is really just TOC’s personified puppet to exercise the Book’s own will.
Actually, Okubo also drew heavily upon his previous manga, B.Ichi, which included the similarly named Nofix.
Now, I have a question:
What went into the thought process behind Gopher?
And I hate to ask, but could someone explain to me why he does he have the fan nickname “Waffles”?