Reasons for my class pairings
I tried to reblog Tostajesus to explain this, but it wouldn't let me reblog. =P
Anyway. Like I said, my class theories are a work in progress. There are a LOT of class relationships, more than just active-passive pairs. The problem is, I'm not sure what the other relationships would be.
See, Classes are fundamentally different from aspects. Aspects are universal constants, representations of the things that make up reality. Players are heroes of an aspect. When Paradox Space prepares to have a session, it chooses players based on the aspects it wants in that session. If it wants heroes of Time, Void, Rage, and Space in a session, it will ensure that the people paradoxically created in that session are heroes of Time, Void, Rage, and Space. But classes are classes of a player. Paradox Space may demand heroes of Time, Void, Rage, and Space, but it doesn't care if its Hero of Space happens to be a Sylph or a Bard. That depends on the person.
...Which is all a very confusing mess that I don't actually have much canon evidence for; it just makes sense. But I think that's why the trolls share a class with their Dancestor, but not an aspect. Trolls and their Dancestors are genetically identical - Paradox Space would ensure that they were the same aspect. But their class depends on who they individually are. Nurture, not nature. Of course, classes are predestined just as much as aspects, but they're predestined to be what they would have been anyway to best fit the player's personality ( see this post).
All of the above was just a very long, rambling way to get to this point: aspects are universal concepts that can have relationships in and of themselves. But classes are tricky and very dependent on the player in question. Basically, I think there are relationships between classes (other than active/passive), but I don't really know what they are.
Now, on to the actual point.
Class being so dependent on player, I looked at relationships between various players in order to determine class relationships. I'll start with the Knight/Seer pair, as that is rather the crux of the controversy here.
Knights and Seers are very much related. My argument for the active/passive pair there is that both use their aspect in battle, but Knights do so actively (weaponizing their aspect to fight with) and Seers do so passively (using the knowledge gained by way of their aspect to strategize and direct their friends). The reason I came to this conclusion, though, is by looking at who was which class in canon. Dave is a Knight and Rose is a Seer. The two are related by ectobiology. But more important in this situation are the trolls. Karkat is a Knight and Terezi is a Seer. And, tellingly, their Dancestors are just the opposite: Kankri is a Seer and Latula is a Knight. It struck me that Hussie wouldn't make such a clear parallel unless there was a connection between the two classes (as he did with the Princes and Bards, or Amporas and Makaras).
Now, the relationship between Seer and Knight could very well be something other than active/passive. But as I've stated before, I don't know what that might be, and there are way too many parallels to just ignore it. Far more parallels than Mage has with just about anything.
As for Mage/Heir, I didn't really base that in any parallels (though Mituna-Sollux could hint at something... It probably doesn't, though). It was more a leftover pairing. See, for the longest time, I had trouble figuring out the active/passive pairs for Knights, Maids, Pages, and Heirs. Then it hit me - Maids and Pages were paired up. Mages supply themselves with their aspect (yeah, Maid might "serve with aspect." Maids are hard because there's basically NO CANON DESCRIPTION OF WHAT THEY DO.) and Pages build themselves from their aspect.This also makes sense because Page/Maid is a male-female pair. That way, we've got one male extreme (Lord), one female extreme (Muse), one male in the middle (Page), and one female in the middle (Maid). I decided that Heir and Knight went together by default, because Heir is protected by aspect and Knight protects with Aspect. But I had some qualms about this.I didn't know which one was active and which passive. On the one hand, Knights do more action. On the other, though, they act for their friends, and the protection on an Heir is for the Heir himself.
But when I paired Knights with Seers, everything fell into place. Heir is passive to Mage. Heirs are protected by their aspect; Mages must protect themselves from their Aspect.
So, that's where my headcanon comes from. Like I said, there are many thigns that are subject to change. I really don't like disagreeing with the Mage/Seer pair, but this kind of seems to make more sense to me. Anyway, I thought I should explain my reasoning here. So thanks for listening to (reading) my rambling!