Amentia

Lee Wee-jin normally endures each of his his high school days as a servant his perfect sister Lee Wi-na. However, he has a secret he can't share with anyone—that he can materialize his delusions as he pleases. When he needs to relieve stress caused by his sister, he materializes a version of his own sister in order to punish her.

But one day a black-haired transfer student says the following to Wee-jin while wielding a knife against him: "You damn delusioner that materializes delusions, we wakers will get rid of all of you!"

(Source: Game of Kekeke)