He is death itself, Death as the most arbitrary final point that decides what lives and what not, or to be exact what is allowed to exist. Death always existed but now is sentient in human form.
Maybe Death was bored and decided to take a break having a peaceful human life but then shit happens and then some retarded Sage isekaied him without knowing.
In the WN the sage that summoned the class curious about MC decides to Isekai the person closest to him, she expected a relative but instead got a scientist from one organisation observing him.
At first the man was scared and cursed his luck for being drawn to the same world MC was but then thanked the Sage for "saving Earth from MC", then scientist guy died because he had nanomachines rigged to kill him the moment he leaks any form of information related to MC.
There was a female assassin sent by said Sage that had the power to see the many outcomes to the future and abuse it to win fights even against broken as fuck cheat characters, but the moment she saw her futures the moment approached MC all she could see was her own death no matter what path she took.
So at the end the assassin lost it, had a break down, BSOD and shut herself in a magic space nobody could invade to escape the slightest chance of MC knowing she exists.