Ernest's condition

From MC's perspective:

I don't know how long it has been. When I opened my eyes. I found that the sky was dark. Then I looked around. Seeing that there was no one there.

I decided to get up. The injuries all over my body were healed including the broken bones. 

Creak~

The old man wearing a white coat entered the room. Seeing that Zack was up. He stepped forward to inspect.

"Hold on, let's see your injury right now," The old man said.

Hearing that I nodded my head at him. After seeing that everything was fine. I decided to leave. 

Right at this moment, the old man seemed to think of something and said, "Be careful. The Class-6 Monsters and above are not ordinary."

"They can remember you. Without having enough strength, it's a death wish to fight against them."

Hearing that I paused my movement for a second. This old man saw through me earlier. How did he find out? Is it possible to guess based on injury?