also, under
著作権法第17条 (Article 17) of Japanese Copyright Law, creators automatically hold copyright the moment their work is created (with no registration needed)
this includes:
>characters (if distinctive)
>stories, plots, and dialogue
>manga art and illustrations
using someone else's work without permission (especially in a way that changes, sells, or damages it) can be considered infringement under:
>著作権法第21条 (Article 21) – 複製権 (reproduction right)
>著作権法第27条 & 第28条 (Articles 27 & 28) – 翻案権・二次的著作物の利用権 (adaptation rights and use of derivative works)
this means even parody or fanwork may be infringing if it uses protected elements like characters or storylines
japanese law strongly protects an author's
人格権 (moral rights) under:
>著作権法第113条 (Article 113) – 名誉・声望を害する行為 (acts injuring the author's honor or reputation)
which means even if a doujin or fan work is non-commercial,
depicting a character in a way that damages the original creator’s reputation (e.g., sexualizing a shounen manga character in NTR without the permission of the author) can be ruled illegal
selling fanworks, especially if they:
>use copyrighted characters
>include sexual content
>or distort the original work's image
…can violate
both copyright and moral rights and lead to
takedowns or legal action
this includes platforms like:
>DLsite
>Booth
>FANZA
>PixivFANBOX (if monetized)
you can read more about this here:
You must be registered to see the links
and you should stop playing the game as well 
