And now, some Wild Mass Guessing
Okay. So everyone in the LoK fandom (or so it seems) is under the impression that Vaatu is somehow connected to Koh, given the visual similarities of the tree Vaatu was trapped in to that of Koh's residence.
Here's my theory. What if Koh was meant to be the prison security system? It doesn't seem likely that Bryke would make them the same being (though I'll be happy to be proven wrong), and given how many millenia have passed since Vaatu's imprisonment, it makes sense that even full-time Spirit World residents would forget Koh's original function.
And, let's be honest, what better way to keep someone from going somewhere is there than to steal their face if they so much as change their expression?
This has been the inaugural edition of Wild Mass Guessing.