I found an item_data.txt, I don't know if editing this file is at all useful, there's stuff on AP and SP usage. SP valued from 0-1000 (1000 being 4 bars I assume.)
Going through the HFS files, the only thing I found on possibly creating items was in the same HFS as item_data.txt called blacksmith_recipes.txt but it did not list all of the craftable items in-game. So I'm a bit confused by this.
If someone could help me out in any way that would be great O_O
*Edit* Also, no luck with finding anything about evil core spawning in the monsters' txt files.