Negotiator

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A negotiator is a headmate whose role is to settle disputes or arguments externally with singlets or other systems apart from their own.

They find compromises where other system members may be reluctant to or too stubborn to even consider. They take pride in finding the "middle ground" that everyone can in some way agree on.

Related Terms
A moderator is a headmate that focuses on intra-system relationship of any kind and could help improving them; and an intermediary is the out-of-system equivalent.

A rationaliser is a headmate who helps their system rationalise (i.e, argue, relativise, promote critical thinking, avoid overthinking or irrational decisions); this traumagenic-exclusive role may assist a negotiator.

History
The term was coined on Tumblr by the Dödstöld Package multiplex.