Resonants

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Resonants are headmates that seem to be made "of the same stuff" and resonate with each other, to the point where had one not formed/arrived in the system before the other, they would have been the same person. They may also be versions of each other from alternate realities, or one may be a reincarnation of the other's past life.

Related Terms
"Symbiotics" is the sentient headspace counterpart of "Resonants"; this type of connection is called Symbiosis.

Fenitive, seelen, and versiomate all describe headmates somewhat similarly connected to each other.

Synpath is a term that refers to having a similar deep connection/bond with something, such as strongly relating to it, acting like it/understanding it, but not quite being it; typically a fictional character.