Mug from Adamantoise(Long horn island) to get whisper,defeat them without Mug to get Adamantine(high LV) or Orichalcol(Low LV)
Fight Blitz(Centra) and sometimes you receive Power Generator (the ratio is about 5%-8%)
There is no way to get Hero and other invisible items except from some boss battles ,refining cards and Chocobo world