That's Sou (the friend, who is narrating the scene) as a kid. The words aren't really there, but this is my takeaway now that I've had some time to gather my thoughts together.
Seiya's mother yells and beats him because he likely looks far more like his father, so that causes her to lash out. So she in turn latches on to Sou who does not, to treat him as the son she really wants (this can even be tied back to Sou's flashbacks from chapter 4).
So we come to here where Sou ends up yelling at Seiya not because he's angry at him, but being angry for him. They've been together for, let's say around a decade (Sou only says he'd been going to Seiya's house every day "since I was little" so I'm just trying to make easy math of that being ~6, and now they're in the middle of high school, so ~16), and he's seen all of these things that have gone on in his life. And despite all of that, nothing seems to bring any affect out of Seiya, even after the funeral when no repercussions would happen.