
Well, it is possible! Pain is different from person to person, but all tattoos hurt to some degree. Just depends on how well the person handles it.
Google "blackout tattoos" and they are similar to what Guwon has.
Some artists specializes in that style, while others prefer to avoid it :p It must be difficult to get it equally black and avoid discoloration

Blackout tattoos are definitely a thing, very painful as you can imagine how many times the skin has to be punctured with a needle for it to completely back out the skin (the process would take hours) It also has some health risks irl, not being able to see rashes and such on the skin could cause problems or delays in treatment etc

As a tattooist I can answer that: yes, it's possible, it's called blackout tattoo and yes, it's VERY painful since the needle used is the most painful one, mostly on the hands. But to be honest I don't think that's tattoo, I would guess he's actually kinda dead? Like half zombie, that's why he has no memories of his past life, doesn't know basic knowledge, is cold as dead and has this appearance (white eyes, "rotten" arms, stitches on the neck etc)
Ahhh he’s falling in love ε=ε=(ノ≧∇≦)ノ
Btw, I’m curious.. So the dark parts on Guwon’s arms from his shoulder to his finger tips are tattoo? Is it possible to have a tattoo like that? Isn’t it painful? I’m sorry if these are stupid questions. I’m not well versed about tattoos.