NPC is a game term. It stands for Non-playable character. In this case it means that the old man doesn't get involved more than necessary, I guess?
Soooo he does not have a specific character?? What is the purpose of NPC??
An NPC is usually a character that is not entirely significant to the story but is there so that the plot can progress with the main character. Here actually rather than NPC, the term used is Mob, to signify a mob character who usually is one in a faceless crowd. Like in other mangas the artist doesnt give faces to some ppl in a crowd=mob characters.
Wow interesting plot....anyone can explain to me what is NPC?? like is he some kind of "love doctor" or something??? ╮(￣▽￣)╭