I thought it would be fukubei too but if he was that, he would have rejected Sadakiyo like he confessed before he died, but it turned out that the kid and sadakiyo became friends. So I doubt friend is fukubei + he had a family to take care of.
Seems more and more likely Fukubei is friend. Especially considering the dialogue they had under the Teru Teru Bozo. Also a very emotional chapter overall. Gotta feel bad for Sadakiyo, really like the callback to when he was crying in front of the student that was beaten up several chapters ago.
