I don't remember exactly which chapter it was but I'm sure it is at the beginning of their arc. It explains the connections between the three friends since they were kids. I remember that in one chapter it explains that Kiriya is not the bio kid of that horrible man (he paid for his school but treated him like garbage). There is a scene in which he goes into the car with this lady and the lil bro ( he was a baby at that time) and the car gets hijacked or something. She decides to leave Kiriya in the car and leaves with the baby. Bitch! That's when he gets the scars.

So, the dude saw his mom killing his dad and kept on living his life with the poor man's body in the garden.... what the f?!!! And then in his mind Kiriya is bad because he is yakuza but his mom is a saint?! What the double f?!!