Revisor

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A revisor is a headmate that has the ability to influence, change or add to another's source.

They may or may not be aware of the ability, be able to do it on command or to revert changes made. For example, a host may be unconsciously changing the canon of another headmate through reading fan fiction or neuronarrating, while others may be able to make revisions without needing to directly interact with the concept of the desired change.