Let's be honest... That was never explicitly stated or hinted at. It's just that his *appearance* looks that way. But unlike teach, we still don't know Guwon's past. Which is why if the author's rushing this I'm going to be pissed off - there are so many questions left and initially they were making it like Gueon just didn't know any better and needed to be TAUGHT how to be human, which made for an interesting psychological and philosophical premise. Now I feel like they're just going to kill them off and I'm like whoa whoa wait

Wait a sec I'm pretty sure Guwon is undead anyways and can't really die?