>>280730261
He's not just a neet, the synopsis I found describes him as an actual hentai addict:
> Self-professed "home security guard" Mahiro Oyama has not left his home in years, secluding himself in his room playing erotic visual novels. This depraved lifestyle causes his prodigious sister, Mihari, to worry about his well-being. In hopes of solving this problem, she devises a plan to rehabilitate him back to normalcy.
> The first part of Mihari's plan is to concoct a medicine that changes her brother's biological constitution into a bona fide female, much to Mahiro's vehement dismay. Stuck in this predicament, Mahiro has no choice but to live out his life as a cute girl until the effect wears off—if it ever does.

So it's basically a hentai-lover's fantasy of getting forcibly transformed into an underaged girl, so that he can be repeatedly molested by other underaged girls. Up to and including his own sister. Rotten through and through, even by anime standards.