What is Seethus actually? What's his backstory?
Hmm, I don't really think we'll get into it in-story, so I guess I'll say some things about that over here, since you asked...
The out-of-world backstory for Seethus is basically that one of my long-time collaborators (hell, the very person who gave me the original idea for Gladiator) suggested this OC for me to use, as an interesting potential ally for Ozai to rely on. I jumped on it, not only because it was an interesting concept, but also because... Ozai needs more allies, haha. It's kind of sad that I basically had nothing to go on based on canon alone. That his most reliable military officer in the show is his fourteen-year-old daughter doesn't bode well for his rule and militaristic campaign, does it? :'D So yeah, the idea of a bodyguard for Ozai, of a sort, came in handy perfectly for my needs.
In the original concept, Seethus had a rather tragic backstory where he had been raised in a very toxic/abusive household. He would have found ancient ruins with old symbols or so (kinda like the Sun Warriors) that allowed him to attempt an interpretation of firebending that went beyond common firebending. I can't remember right now if it was in the original outline or not for this to be lightbending all along, but I think it was? Anyway, the point is it didn't really work out as intended, something about the recipe kinda backfired, so to speak... and Seethus ended up in a rather twisted sort of "undead" circumstance where his otherworldly powers are also a disease that is killing him, much as he can use it to his benefit in combat or stealth missions.
That's as much as I can say about his origin story, to a fault. As I continued to write him, I concluded that Seethus did indeed attempt lightbending (I kind of used the Head Sage to convey those theories/explanations), but it didn't work out for him. Kind of like how lightning is supposed to take a toll on the bender if it backfires, lightbending, as I conceived it, took a rather steep one in his case. Basically, the misfire twisted the nature of his very energy, his chi, and corrupted it without outright killing him. It's as if the putrid stuff Azula was ailed by, after the Spear's essence infected her, could become her weapon...?
In Seethus's case, he lives off a very delicate and dangerous life-death balance, where too much life, too much humanity, would cause the corruption inside him to consume his remaining life-force and kill him. Too little life, and he'd die anyway since his life-energy would stop flowing and the corruption would spread and seize him completely. The main thing that makes him dangerous is the fact that he can bend this corruption, whereas Azula never did/tried, as her situation was a loooot milder than his.
So, Seethus is human, not a spirit. It's entirely possible to say that some spirit caused his bending misfire and corrupted his energy when he attempted to lightbend, of course, but I don't really have a full-blown answer for this because... I figured a lot of his appeal as a character was precisely about not having absolute, precise explanations as to who and what he is. Personally, this is how I envision him and the root from which I write him, but you're free to headcanon whatever you'd like about who he is, and where he came from, seeing as I'm not really going to go in-depth about those things in the story :'D
