Posted by: @agendavolo

But we are talking about some name-surname that should not be visible at all.

I don't think this is the name surname, I think this is just a nickname built on the name and surname. In any case, I have nothing to add here. Maybe you have some page where all users are listed, but it's not wpForo page for sure. wpForo only shows display name and nickname. It doesn't show first/last names if you've not displaying them on the profile page using the User Custom Fields addon. There is no other ways to get that data from wpForo, so this question should not be related to wpForo.