After looking at past chapters:
@tprat is probably right in that the SS kid is probably Kiriya.
The right age in the past/present, same hair, same eye color, using guns, past connection to Great, Great in the present time implying present Kiriya holds back a lot and that he lets some criminals live instead of killing them(easy to see where he got that from), ect.
It would actually be odder if SS-Kid
isnt little Kiriya.