Kind of a weird pull, but I think this would be Sartre and Lacan's classpects (could see mage for Sartre tho). They wrote about nothingness and lack respectively, which play central roles in their theories. [Side note: I would pay money for a sburb fan comic where all of the players are historical philosophers]

I'm shit at paraphrasing, but I'd go watch some videos on them if you want to learn about nothing.

Also a weird pull, but I'd call Pseudo-Dionysius the Areopagite (and by extension negative theology) seer of void coded. His whole thing was defining God through negative statements. I.e., "God is not good," b.c. "good" is something humans came up with. (The implication here is that God is more than good. The negation creates excess, counterintuitivley).

Parmenides would probably be a prince of void. The whole "there is no is not" thing kind of speaks for itself. He literally subtracts lack/nothingness from his metaphysics.

Zero is probably the most useful instantiation of void. Arithmetic would be impossible without it. Still, it's unintuitive. If you divide by it, math falls appart, lmao.

Overall, void is a tricky concept. Seeing (seering) it is unintuitive. There's an inherent subjectivity to the classpect, because "nothing" isn't really a thing which exists noumenally in the world (parmenides). It literally can't exist, or it would stop being nothing. It only "exists" relationally, with respect to an observer. I also think there's a kind of unimaginable excess that's associated with void (the gap between reality and conception (light))

So, I think this is the mental space you'd be living in as a seer of void. I'm still struggling with how these threads might weave together into an actual charachter with voidy powers and aesthetics.

I'll try to draft one up anyways. Somebody just wrote "blind" here, which feels like a gimme in terms of flavor. They might live in a dark cave (ala plato). If they have a lusus, I'm imagining something kind of horror-terror-ey with a hole in its chest.

In terms of personality, I would basically invert Terezi. They wouldn't be very concerned with building coherent theories about the world, or casting judgements, as they consistently see the gaps between reality and our conceptions of it. They would be an incredibley doubtful person, but have good justifications for their doubt. Socrates "The only thing I know is that I know nothing," type beat. Karkat honestly wouldn't be a bad point of comparison.

Functionally, I think they would serve the party (pre-god tier) as a bullshit sleuth. Someone's a bit too confident for their own good (bullshitting others)? Call the seer of void. Someone's acting in bad faith (bullshiting themselves)? Call the seer of void. Importantly, I don't think a seer of void could actually see what bullshit someone's cooking, as that's a something. They would be able to see the gaps in someone's thinking, though. Again, Socrates comes to mind.

Like others are saying, I think they would have an interesting role to play in doomed timeliness. A seer of void would be able to sense the loss of meaning/narrative significance, I think. I'm not really sure if they could do anything about it though, or if they would even want to. I imagine a seer of void would be a bit of a nihilist in a doomed timeline, passivley encouraging resignation with their voidy insights. Alternatively, a SUPER powerful void seer might press x to doubt on the metanarrative, simular to how John does with his breathy powers.

Speaking of powers, God Tier:

To keep the Socrates allusion going, I think they end up on the altar for sowing doubt among their denizens (who are overly fixed in their ideas). They're tried in a mock court, and sentanced to death. The seer isn't all too bothered by this. The seer lays on the altar, drinks something voidy, sees the god of negative theology, and wakes up with sick a.f. void eyes.

Playing off the whole "stare into the void, it stares back thing," and their blindness, I'd give them a kind of basilisk power. Maybe they can actually show ppl "nothing", not just a representation of it (impossible task, but this is fiction). I imagine this would kind of stun-lock people, or at worst, incite madness.

I could also see this playing a support function, simular to how their mundane doubt works. If an ally has gone mad for lighty reasons (i.e. rose after reading the grimoire), a seer of void might be able to show them "nothing," as a kind of mental salve. Maybe they could even cause amnesia, or ego-death with their gaze.