Cassi :D

"Not too important not too unimportant" is very specific wording. We technically can consider the existence of more humans as not extremely major to the main plot in the same way as undertale (at least for now), but still important for kris' character giving their species dysphoria (shutting the "how to care of humans" book when they see photos, the horned headband they wore for months, etc)

It's also very specific that:

They are the only one who has access to the room and interacts with the man regardless of who is in the party

Ch3 with the man saying they have forgotten him multiple times before

Ch4 with the art therapy, and all they could draw was a smear that eventually a tree was drawn above everything

So yeah, my theory would be relating to the idea of kris repressing the idea and the memories of humans due to how distant they feel from it, eventually covering it up in their head with an object, even tho this doesn’t truly go away

Of course, there would be still holes about it like:

What exactly is the man in the dark world in the first place

Why an egg (although you can say it's a symbology for creation and all)

Why it can be brought in the light world and put in specific places

Why The Voice That Is Most Likely The Funny Man treats the eggs as an "issue"