Anyways!!!!! Etho is the void itself which has just been molded into a human-esc shape, he sometimes struggles to keep everything in order, which is why he has the mask, there isn't anything under it. Just void. As just as much as he is void, the void is him. They are intrinsically linked, they are one in the same.

Also, noone knows anything about Etho being void, they just think he's like a hybrid of some kind as he kind of just brushes the topic off it comes up. This has caused lots of situation where the hermits spy on Etho, trying to figure out what he is. (they havent)

Joel somewhat ironically knows more than most people do, seeing how their souls were linked for a period of time. During that time some of Etho's void traits seeped over to him and he had to ask Etho how to hide/control them. (cue hours of Etho helping Joel control his void)