Some people might call them helpers, others tagalongs, I'm talking about NPCs that follow the player and give a stat boost on certain quests.
It seems like all the ones in Polaris are set to just follow the player as long as they're in Polaris, and not care where in Polaaris they go. For instance, Leon was probably not meant to be able to leave Walruskberg, but I was able to bring him all the way to Frigid Maw. The biggest issue, however is when Ivan joins the player at the beggining of the Imperial Palace dungeon. Once you pick him up in the first room, it's possible to take him out of the dungeon and pretty much keep him with you forever.
