Matter
In headspace, we are made of an abstraction of matter. This is important enough to us that we want to share how it works.
You could say it matters a lot—
It has infinite resolution and can be scaled up and down freely. Whereas real matter is made of microscopic particles, this stuff isn't comprised of anything, not molecules or atoms or quarks. It just exists!
By default, the substance is completely neutral. It starts as a medium-gray semisolid that might seem similar to clay. However, the only property that it truly has in this state is whose it is.
Some of us call it mindmatter because a mind can claim any amount of it. After creating this link, they can shape, color, and adjust other properties of their new physical form however they like—full bodily autonomy. Their personhood is stored simply and perfectly at every single infinitesimal point in their form. Their body won't do anything that they don't want it to.
Naturally, an embodied mind can shapeshift at any time. You know, like how things so obviously should be.
Multiple minds can agree to share the same matter. This lets them fuse into a single being that is a composite of their traits.
The matter can convey sensations like flavor. For organic beings, it bypasses the usual receptors, as there are no molecules to sense. It shortcuts straight to perception with no risk of causing chemical harm. It can even convey qualia beyond what the beholder would usually be able to perceive.
We like to imagine that even seemingly inanimate objects in our headspace are made of claimed mindmatter. If we have a sofa, that's a piece of our mind that is fully existentially content with being a sofa. We're cool with that! There's probably a lot to be said about our neurotype and animism, and it's doubly applicable within our own mind.