A categorist is someone who is deeply into categorism. For example, a racist is a categorist focused on race, while a sexist is a categorist focused on gender.
Caution is advised when it comes to calling someone a categorist, or any of the specific foci of categorist such as racist or sexist. Categorism is usually something that people do, rather than something that people "are" as in having it as their identity or as some sort of "true essence". Everyone do categorism to some extent, although most people try to be fair and minimize it. In most cases, it is far more reasonable and constructive to claim that "your behavior or views in this case here and now is categorist" rather than to claim that "you are a categorist person".
It would be dichotomism to claim that every person is either a categorist or not a categorist. Instead it is a sliding scale. Where the line should be drawn is open for debate.