oh hey good thing i checked my inbox because this is a fascinating question

It seems like based on the background we got from A Walk in the Dust that they're like, distinct entities but lowkey highkey soulbonded, somehow? They can definitely feel each other's emotions and they're individual enough that it feels like they legitimately care about each other.

This is maybe just my headcanon, but in some ways I feel like Mon3tr ends up being kind of a heat sink/outlet for Kal'tsit's anger and less controlled impulses—like, if Mon3tr is aggressing at something, it's because Kal'tsit does not like that thing. Sort of the id to Kal'tsit's ego+superego. Kal'tsit is a very tightly controlled person and very deliberate, so most of the time she still exerts very fine control over Mon3tr even when it's a dire situation and denies both of them the satisfaction...

...but also sometimes Mon3tr slams W through like five walls as a treat. She was having a very stressful day.