Well, they're still human... ish. They have all their original memories, which is a relief, and don't appear to have any Hollow features beyond the same kind of eyes that Ichigo sports when his Inner Hollow is at the wheel, and maybe bone accessories if I'm looking at those pics the right way. So... Half-Human/Half-Hollow? Fullbringers are mortals who came in close contact with Hollow Reiatsu while in their mothers' wombs, but this seems a lot more even between the two portions than Fullbringers are. If FBs are 90% Mortal and 10% Hollow, then this would be 50/50.
I think we need to look at it from a few perspectives.
1st: physically they are 100% hollow/ghost/unspecified etheral being. Or well, technically, physically they 100% aren't. This is still hueco mundo one of the worlds made up of spirit particles. Currently they have no atoms, molecules, muscles, flesh or dna as the human world sees it. Though they have the spiritual equivalents of them.
2nd: appearance wise, I agree. To me they look like the hollow versions of themselves a la zangetsu (including funky skin color). They have similar enough bodies that any differences are hard to notice so I wouldnt be surprised if they had no trouble using a gigai (though a standard one may not work since they aren't shinigami). They also don't have masks or visible hollow holes which makes me think that they might not be hollows and are rather manifested pieces of weaver. Something like what Rudbornn Chelute does in canon only slower and with less useless minions. (We did eat him after all, well probably)
3rd: mentally they certainly seem human or close enough. Their reactions may or may not be weird, depending on how much they saw/understood before being reborn and how much is a coping mechanism. They definitely don't act like normal hollows though.
4th: spiritual makeup/biology (ectology? ectobiology? No wait thats homestuck) this is the one we really dont know. In this story we have encountered hollows (normal and 3 levels of menos), shinigami, dead humans, empowered humans (fullbringers and quincy) an unknowable eldritch abomination (nightgaunt), a sapient self limited celestial body (nickname pending) and we have been told that we fought and killed a god and have a few memory fragments of it. None of these quite fit the children though hollow is closest.
Rampant speculation. (As opposed to orinary speculation from above) My pet theory is that they are both technically hollows without hollow holes. Because we safeguarded their hearts even as they became hollow (in the sense of unlocking that form of spiritual power) they avoided the standard side effects. First the chain of fate being removed and loss of heart. We had their hearts kept them and put them back, filling their holes. Second breaking into particles and reforming in a monstrous form. They were broken into constituent pieces inside the menos but when we remade them we wanted them looking human (unconsciously since we are trying to save them). Third the mask. From what I remember the mask is a shield against the world to hide due to disgust at self or world or lack of memory or a hundred other things. Having preserved their minds, preserved their bodies and restored their hearts they would have no need for a shield between themselves and the world, and thus no mask (though they might be able to make one if they felt like it).
