So, a long time ago I've read this manhwa and I can't remember the name anymore but I do remember some things about it
- The protagonist had a sister and he had a sister complex (that is, he liked his sister)
- His sister was blonde
- He had some sort of special ability of turning his imagination into an almost real illusion, kind of like Itachi from Naruto
- His sister treated him badly so he uses his special ability to imagine a dream-like version of her (one that likes him and acts in a cute way) and bring that version to life, so he can just be with that version of her
- It seems he's not the only person in the world who has this ability. There are others around the world. One of them was an otaku guy who uses his ability to bring to life some sort of anime/videogame female character he likes, or something like that
- Apparently that ability that the protagonist has is actually dangerous and there are people in the world using it for evil

So... Manhwa names anyone?

Amentia?

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~4400 manga completed
~1700 ongoing...

Oh, that's it! Thank you so much!