Attention, spoiler.
“Once it becomes a calamity, there is no going back.”
In a world where Nier is everything, Un, the son of a deserter, suffers from Shiro syndrome, which makes him unable to use Nier. In a world where even the richest have few servants because they can clean their houses with telekinesis, how would he get a job?
Despite all this, Un had a dream: to help his sister enter Nier's academy. Even though she had no talent, and her ability to manipulate Nier was below average, she was hard-working. However, a demon appeared to Un and informed him that he had no soul, and therefore, he could not reincarnate and that this was his only life, and on top of that, his sister would become a failure because her physical core of Nier had cancer, which would not kill her, but He also offered a proposal:
"Kill your sister, absorb her soul, and I will make your current single life worth living."
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In other words; if you kill your sister and interrupt the cycle of reincarnations of her infinite immortal soul, I will grant you the opportunity to make your one finite life worthwhile.
Sacrificing an immortal soul to make the life of an empty body worthwhile.
***
About One, or MC.
The world is full of neutral people, who are neither very moral nor too immoral, they just choose what will benefit them the most (within certain limits, of course, most people are not psychopaths). In other words, I am not warning you that the MC is not a moral person, nor too moral; he is neutral; he does feel guilty about certain acts, but he is willing to kill anyone who directly threatens his life on a large scale.
The MC will be too human, will have strengths and weaknesses (although he becomes too strong), and will also make some mistakes (rare, but the script cannot hack too much even if the world revolves around him, so it has to be nerfed a little, right?
In fact, the world will not depend on the MC to turn it around; no matter how much the MC becomes a calamity and the world persecutes him, the world will not depend on him; rulers actually make logical decisions, and each kingdom has its own particularities.