Early on in Final Fantasy XV's "Let Sleeping Mountains Lie" quest, you visit a location where a bunch of NPCs you have met throughout your travels gather together to help you out. (I previously said this was the first thing that happens in post-game content, but apparently that was a mistake. It's just common, not the actual quest trigger.)
When I got there, the game told me I had met "13 of a possible 14" of them. Of course, instead of thinking "awesome, I did a pretty good job," I thought "who did I miss?!" So, who are the 14 people you can meet there in the diner? I tried looking it up, but the guides I found just gloss over this, saying little more than "a group of NPCs exists, visit them" before skipping ahead to strategy for the more complicated later part of the quest.
As a follow-up, what are the conditions for making each NPC appear (I know it's not necessarily "complete the quest chain" because at least two of them still had more stuff to do) and what do they give you when you're there?