Incorporeal
"Hmm. Okay I understand, and you are right. We do need each other to survive, so to speak, so there is no reason to hide anymore. Why don't you reveal yourself and we can talk more about these hunters?"
Hao Xuan took a couple more steps back to give Ryuk some breathing room but it was a useless gesture because-
"I can't."
"Why not? You still don't trust me?"
"I don't," Ryuk gave a spirited headshake, "But that is not why. I have no control over it, we are born this way."
"You are born...invisible?"
"Not invisible, incorporeal. Us observers are born of the time energy, which is why we feed upon it. Without it, we would become weaker and weaker before ceasing to exist altogether."
"Huh, that's a new one. But then why can I see the rough shape of your body?"
Ryuk shrugged, "How would I know?"
But Hao Xuan already had a pretty good idea.