So what is sougen? we know he's not a human but we don't know if he's a monster or not too...

My lucky guess is a mountain spirit/god's child. He doesn't age (he's more than five centuries old now) even though physically human and more than strong enough to break a nine tailed fox's barrier without effort. That's not something ordinary youkai can do even at best. (〜￣△￣)〜

But what about when he goes to sleep for long periods of time?

I think, he's another fox, as strong as Shinoda. Because Shinoda said that he looked familiar when he borrowed his ears and tails.

He also said he died a few times (he could just be joking and didn't actually die from simply starvation).

I tried Googling yokai that sleeps a lot. No result. I think him sleeping a lot has got something to do with his half-human part.

Tysm!!