Stopped caring about this ever since a certain something happened, but it's not enjoyable to see @Cynical get outnumbered in his conceptual all fiction argument:
Erasing something conceptually doesn't mean it gets erased from memories, much less in Kumagawa's case. Be it ligh spectrum or conceptual it will always be erased from memories cus it never existed, it never had a cause for existing. But read my blog if ya wanna know why "they don't forget". As for proof it was a conceptual erasure. Ezumachi's skills didn't work anymore, if he had only erased the light spectrum then his abilities would still work because "blue did still exist" it just wouldn't be visible for ppl. Ezumachi's skills were stated to be worthless after the erasure, without colors there can be no manipulation, wouldn't happen with ligh spectrum, oh did i mention that Ezumachi's skills affects the idea behind the colors? Yeah... Then the tube that should have blue paint was empty cus it didn't exist anymore, im sorry, it never existed, it would still be there if he had just erased the light spectrum. Need i go on? Colors were a concept.
He erased aura...a concept. Inb4 someone says "aura is not a concept, cus it can be tangible", it's not tangible in Medaka Box, it treats aura as the concept of "remembering". So unless ya wanna argue that Kumagawa mind haxed the world, then he erased a concept.
He stated to be able to erase combat...concept.
Also about the erasure of ppl...how can someone have a concept if his birth was negated. He was literally never meant to be part of the world, he "never happened" are you seriously gonna argue his concept exists? Cus that would mean his concept exists independed of him, i mean there would be concepts for ppl who will never be born.
Then again, to go ahead, he affects stuff like pain, fear, emotions, damage, importance, somehow affecting the ability to die (when he literally stabs his brain for fun, without caring that it should kill him, i still have no idea what he erases though) etc just randomly for fun.
Again this thread can be closed idc anymore, i just disliked seeing him get outnumbered in a debate.