Purist

A purist is one who desires that an item remains true to its essence and free from adulterating or diluting influences. The term may be used in almost any field and can be applied either to the self or to others. Use of the term may be either pejorative or complimentary, depending on the context. Because the appellation depends on subjective notions of what is "pure" as opposed to "adulterating" as applied to any particular item, conflict can arise both as to whether a person so labeled is actually a purist and as to whether that is desirable.

According to the Merriam-Webster Online Dictionary, the term dates from 1706 and is defined as "a person who adheres strictly and often excessively to a tradition", especially "one preoccupied with the purity of a language and its protection from the use of foreign or altered forms."

In entertainment, a purist is a person, gamer, or audience member who considers modifications to a particular entertainment item unnecessary or even offensive, vehemently so if against the specific wishes of the item's creator. They also may make it a point to correct fanon, which they stereotypically detest.

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Wikipedia