@the_white_death
Everyone Else is a Returnee.
It's a litrpg. When tha Akashic Record makes Earth part of the System, the world is frozen in time for 10 years so Earthlings can be sent to other worlds for that time to pick up the ability to use mana.
The main character is so unnoticeable that he literally gets forgotten and left on Earth for that time. Due to the error, times slows instead of freezes, leaving him alone on Earth for 1000 years with only the company of a single angel. The angel rarely ever shows for the first 50 years or so, so he begins learning. Everything. He learns languages, fighting styles (the angel is a combat type, so teaches him spear and vale tudo), any weapon you can think of... Then he learns how to blacksmith and dismantle, because he knows those will be very valuable in the new world.
When time finally resets, he's a terrifying monster...that can't use mana. The time freeze and the thousand years of training cause his body to evolve, making his level 1 stats a bit nuts.
All of this is in the first five chapters or so, lol.
Yu Ilhan is so hypercapable that "Mary Sue" doesn't even begin to cover him.