@Avenger88id
Everyone Else is a Returnee.
It's a litrpg.
When the Akashic Record (The System) activates on Earth, Heaven freezes time on the planet and sends all the Earthlings to other worlds that already have mana/the AR for 10 years so they can utilize mana and survive the monsters that will show up on Earth.
The main character Yu Ilhan (his name literally means 'unique') is the only person on the planet who gets left behind, lol. You know that person no one ever notices? That's him. Except his unnoticed nature is so bad that even God didn't see him.
Because he was left behind, the time freeze screwed up. Instead of Earth being frozen for 10 years, it instead moved incredibly slowly (1/100th) and caused him to be left behind for 1000 years, with only a single angel as company. So he decides to train for the time when humans come back, having no clue that he'll be there longer than 10 years. He travels the entire planet. Learns every language, masters every major weapon type (the angel teaches him the spear and Val Tudo, and he learns the rest from there), and masters blacksmithing and dismantling to absurd degrees.
That's the rundown of the first three chapters, lol. And it gets stranger from there.