From Pluralpedia, the collaborative plurality dictionary
controller (n., adj.)
Applies toheadmates
CoinerDust! Sans from the Splitting Hypnotizer System

A Controller is a headmate who watches over other headmates that might harm themselves and/or others. This is in order to stop those headmates from either doing harm or doing more harm if already done so. They can also be assigned to certain headmates.

These headmates can usually be paired with masochist, sadist, persecutor, or self-inflictor.

This role can be assigned but would rather be chosen.