Beings that appear to be the same as the protagonist, often with faint or even quite noticeable differences. Often these are an evil clone or twin, or some manifestation of the Evilness in the protagonist, or even almost literal mirror images of them.
Alternate names: Evil twin, Mirror image
Name variations: simulacra, doppelganger, doppelgänger, homunculus, homunculi
The first video game about Simulacrums was released on December 1988.
Mirror image refers to a somewhat common instance in fiction where one fights an almost literal mirror image of themselves, any wounds being "reflected" upon themselves and such. Commonly the only solution to beat such is to sheathe your weapon and pass without aggression, though there's room for alternate case where you must defeat the simulacrum with violence despite the indication that it would be fatal to you too (essentially overcoming a perceived fallacy that you're fighting yourself rather than someone else, inverting the initially described case).
Unlike doppelgangers, simulacrums are unable to change their shape (by default, usually only able to do so if the protagonist can too) and their appearance is not maintained for the purpose of misleading anyone, rather it is "natural" state for them.