Thief: Someone who steals their aspect from others to use themselves, or someone who steals from others using their aspect. They can do this consciously of their own free will, but it also seems like it comes instinctively for some thieves. The fact they can steal their aspect is significant to them in some way.

Void: Nothingness, absence, a lack of information, and darkness, but it’s also a blank slate with infinite potential. Void players can hide themselves especially well, and excel at avoiding observation from those they don’t need spying on them or escaping when they’re in danger by slipping away. A silly example would be having a slenderman type effect on surveillance cameras and such. Escapism is heavily associated with this aspect, as every known member of it has some sort of vice.

So, the thing that usually makes it significant for a thief to be able to steal their aspect is that they lack it. Vriska had shitty luck for a long time and had her magic cue ball was broken, and Meenah "saw her future role as the Queen as nothing but a form of enslavement." So a thief's aspect is usually something that they feel was stolen from them, first.

Writing all that out helped me figure out a way this could apply to a Thief of Void; they could be someone who's constantly under surveillance and lacks any kind of privacy at all, always feeling like someone or something is watching them. Maybe their guardian is abusive or overbearing, they could even be in a situation similar to the Handmaid. The escapism that's common for void players could be more literal in this case, because their goal is to literally escape from whatever is always watching them.

As for their powers, in a narrative sense, if a Thief of Void wanted to be a less important character, they could possibly steal the narrative irrelevancy from someone else and take it from themselves; An unimportant character takes on what they were doing, while the Thief fades into the background of the story. They wouldn't have to do this, it's just something I think they could do.

In a literal sense, them stealing void would be them always knowing where someone or something is. No one or nothing could ever stay hidden from a powerful Thief of Void. It's a simple power but if utilized properly, would make them a valuable asset to their session and very good at playing the game. The more they steal void from others, the better they are at hiding themselves, and could use that power to steal whatever they want from others. They would not be able to hide anyone else.

There's possibly someone who might wonder if a Thief of Void would have powers much more similar to a Rogue of Void, however, Rogues are ones who steal directly from their aspect, and thieves are ones who steal their aspect from others. So unfortunately a Thief of Void would not have powers nearly as insane as Roxy does.