@thomas4721 It's not just limited to Japanese mythology. Controlling a spirit, demon, oni and whatever with their true name exists in many other cultures and mythologies as well.
@DirkHardpeck put it best. If someone is angry because it wasn't what they expected. The manga was always a coming of age story that plays on the old adage that true personal growth happens unexpectedly and not from knowing the future. If that flew over your head then I got no idea what to say...
@theroadstopshere This is based on the LN version of the story. There are already massive differences between the WN and LN versions in the first volume alone.