alicetama wrote:Who knows. If they get permission from Gosho, it can be, or if Gosho himself writes the script. An example of this would have to be the characters Filia and Martina from Slayers. Kousaka said that they were both canon, even though they weren't in the novels. (Then again, this was a decade ago, so...

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It doesn't matter if the creator likes it enough to "declare" it canon or not. If they don't actually include it in the source material, it isn't canon. The only time this can be circumvented is if the source material is based on multiple realities/timelines/etc that would allow it. Filia especially can't be canon, since her entire season is based around a plot point that is impossible with the books.
If the author wants it to be canon, write it into the source material. That's what Haruhi's writer did with "Someday in the Rain." It was such a popular story in the anime that he did a short-story of it for the novels.
Also, it's Kanzaka, not Kousaka.