To unlock this secret shop you must have completely filled out the collector's book in the game (which requires two playthroughs) Once completed, talk to the man dressed in purple at the library in the Daath church to receive Jade's "Item Collector" Title. Talk to the man again with the title equipped and the shop will appear near the inn in Grand Chokmah.
Unlock the following bonuses by performing the corresponding tasks:
BONUS - HOW TO UNLOCK Game Record - Available at the title screen after beating the game Grade Shop - Save a clear file after beating the game, and you will be able to start your new game+ file with the grade shop with your cleared file. Sound Test - Available at the title screen after beating the game Unknown Mode - Complete the game once Very Hard Mode - Complete the game once
The Abyss Replica Facility is a secret dungeon that is unlocked when playing on your second playthrough. To access it, you must have saved Shiba and completed the Ortion Cavern(East) side quest. Go to Sheridan and speak to the two men in the northwest part of town, then Shiba, and you will finally be taken to the dungeon.
Guy is generally considered better, but not by much. He is quicker in his strikes and movement, but may do less damage in bursts than Luke. He is also better at stealing and has better self-supporting moves i think.