The strongest Reborn
Reborn Zombies didn't feel pain and they were able to heal themselves from extraordinary wounds. There was a limit to what could be done with their healing, but in most cases as long as the body part wasn't missing and it was nothing to do with the brain, they could heal it.
However, what Zain could do was completely different compared to every Reborn they had come across, because he was able to heal on the spot, without having to consume human flesh. Healing did take from Zain's stamina and he would have to consume flesh if he wanted to constantly heal.
The difference was, in a way he could borrow from his body's energy reserves to force his body to heal there and then, and in that way, he wouldn't be so disadvantaged in the fight. This was why the instructor Wix ended up so surprised.