PCs are player characters that flood the Root Town(s) as well as can be found in fields/dungeons. Simply talking to them, initiating the menu for talk, trade, etc., is all that you need to get this trophy. You might actually achieve this early on by checking out the trade options some of the PCs have early on, but you will definitely acquire this if you intend to finish the Book of 1000 early on in the first volume (as some items are more easily obtained via trade rather than by hunting them down).