the tattoo in earlier chapter and this chapter is depicted on different hands (first left, then right), only difference is in that curve pointing toward elbow or the fingers.
@randk2k1
the creator is Korean, so I'm assuming the characters and setting are Korean as well.
It's just that the characters were given japanese names for the translation.
The trope of unmonitored bullying/fighting going on in schools is big in a TON of webtoons/manhwas, the reason being that bullying is a real issue in Korea.