Tengu. The name was given to the boy by Yao, because when he met him for the first time, the boy did not need a specific name and was satisfied with the name "the monster of Shormush (city)"
He looks about 13-14 years old at the time of their meeting.
"A being of uncertainty, doomed to hesitant existence between the world of the living and the world of the shadows"
Born among beings who have a physical (material) body, living will fall asleep of the accumulation of the energy of the abyss. "Border Guards" are creatures capable of interacting with the world of the subtle plane of the abyssal element. Their bodies are physically stronger and more resilient than others, but their level of awareness of the world around them leaves much to be desired.
These guys almost never study on their own and live, rather, like a wild primitive tribe. It is worth mentioning that individuals do not gather in groups and are scattered across the continents.
Their nature has played a cruel joke with the bodies. The more the energy of the abyss accumulates in the body of the wearer and he uses it uncontrollably, the faster the vessel itself wears out. The average life expectancy is 26-27 years.
Very emotional and aggressive, violent and out of control. Females are taller than males.
Why did they appear at all? According to the plot, Yao will ask this question, but only twice will he meet representatives of this endangered species.
Abilities are not really known.
Pedigree even more so. The boy does not know his parents, simply because these guys are not particularly interested in raising their generation.
Tengu, before meeting Yao, was engaged in fraud and fistfights, simply because he was only strong in this. But the surprising thing is that the kid had amazing ability to instantly analyze the situation and a good imagination. Plus to everything, a fairly large vocabulary, which he picked up in the port settlement, where many different hermits, merchants and other interesting natives constantly wander.

What will happen to this boy?
what role will he play in the future of the world and will he conquer death?