From Pluralpedia, the collaborative plurality dictionary
border (n.)
Applies toheadmates
CoinerThe Wool System

Border refers to a system member, whose function or role is to intervene during system-wide or individual headmate spirals, panic attacks, splits, or other highly risky emotional and cognitive states.

They may front frequently as a precaution when a system is prone to breakdowns, or may tend not to front until they are direly needed and called into action.

Border is intended for use by plural systems with Borderline Personality Disorder or traits, but is not limited to this application.