Canon

Canon  is the material accepted as officially part of the story in an individual universe of that story. Events and Characters who are considered canon are likely to have an effect on the overall continuity of the Universe. For D.Gray-man is considered canon the material which is produced by the Author Hoshino Katsura.

Canon
Below are the media whose content is purely canon:
 * The Manga: Written and drawn directly by the author, the manga is the backbone of D.Gray-Man. Everything that happens in the Manga is canon. This also includes the materia not included in the chapters such as the Talk Corner.
 * D.Gray-man: Reverse: Written with the collaboration of the author who also provided illustrations for the novels, D.Gray-Man Reverse is considered canon.
 * Databooks: Providing extra informations, the Databooks by Katsura Hoshino give canon datas.
 * Author's interviews: During some interviews, Katsura Hoshino has provided some information about the manga.
 * Artbooks: The artbooks contains illustrations drawn by the authors. As a consequence they are considered canon even though they generally don't have any effect on the storyline.

Non-Canon
Below are the media whose content is either completely-non canon or only partly canon.
 * The Anime: The two anime: The first one and Hallow are considered canon as long as they follow their manga counterparts. They may allow themselves to slightly modify the manga events, or add additional ones. In this case the anime is non-canon.
 * The Video Games