The device theory
Just a quick theory at 3am, probably easily debunkable
Couldn't the roaring knight be a "device" made by gaster?
More specifically, a device used by a monster (instead of an armor) like the orb in the middle of the knight (they even use it to travel faster)
In undertale, we see that gaster is trying to create synthetic souls/determination to break the barrier, a study continued by alphys
In deltarune, the "survey" could just be gaster testing the device, which could either be the synthetic soul itself, or a lure for a real one.
As for the knight's identity, i'd say it would make it more likely for it to be:
- gaster: controlling the device from the shelter
- vessel: taking the synthetic soul as their own
- goners: maybe if they're turned into servants? Idk about this one tbh
PS: i know there's some people that believe ut and dt are unreleated, but afaik it's never denied that they're connected in some way