As a fan of manga and anime, you might be familiar with the term “doujinshi”.
A doujinshi is a self-published manga by a usually non or semi professional manga artist.
The word contains the kanji for 同人 (doujin → “same” and “person”) and 誌 (shi → from the word “zasshi”, “magazine”).
There are usually two types of doujinshi:
The ones where people create original characters and stories, maybe trying to become a mangaka in the near future or because it's their hobby and they want to show their creations to others,
or the ones where people use already existent characters from already published stories and put them in new situations for their own manga ideas. It's important to mention that not all doujinshi are R18, like some people believe. Doujinshi can have various themes and topics and erotic is only one of many genre.
The “doujin” in doujinshi usually refers to a group of the same people, who want to see their beloved characters in specific situations or interact with specific characters – this can even be a crossover between two or more different manga, for example Card Captor Sakura fighting a demon from Inu Yasha together with Deku from “Boku no hero academia”.
Some already published mangaka in Japan come from the doujinshi scene and sometimes even started with copying characters themselves. Others created and self published their original works to get attention and were finally recognized by the big publishing houses due to manga contests or because their works gained a fan base in social media.
There are easily hundreds of conventions in Japan per year where you can book a table and sell your self-published manga. The most common known and biggest of those might be Comiket. The amount of people trying to get attention for their work there can be easily overwhelming.
At stores like “Tora No Ana” or “K-books” are so many doujinshi stacked in the shelves, I remember nearly having a heart attack when we visited those stores in Ikebukuro and I found the section for a manga (and a specific pairing) I went in there for... I lost a lot of money on that day.
In America or other countries in the world, it seems impossible to just “steal” someone's characters to publish a book with them to gain profit. Lawyers would sue this person immediately due to copyright laws.
In Japan however, there's a little loophole in the law. It's not exactly allowed to use someone else's intellectual property for your personal enrichment but it's important to explain why publishing houses and professional mangaka don't sue the creators of doujinshi.
Well, actually, there are several reasons.
One of them might refer to the fact I explained before: A lot of today's popular mangaka used to make doujinshi themselves – so who are they to tell the new generation of creators to stop doing it? Plus, their reputation and the one of the publishing houses would be tattered if they sue their biggest fans. And you never know, maybe the artist they'd sue today becomes a great mangaka one day and that certainly wouldn't be a great advertisement for the publisher.
Talking about advertisements – a manga can get a lot of exposure through doujinshi. Some readers, who like a doujinshi without knowing the original work, may start reading and buying the original manga series. That way doujinshi can create new customers and fans of a series.
Also, “borrowing” a character from another work for your own has kind of a history in Japan. It already happened a lot of times in the past, even with Disney characters.
So as long as the creators of doujinshi don't get too much attention or earn tons of money from their work (which they mostly don't), the publisher and original mangaka wouldn't sue them. Usually, self-publishing isn't the easiest way to get rich, especially when you sell your work in print at conventions and not online exclusively.
If you like doujinshi and read them yourself, make sure to pay for the artist's hard work and don't use websites that upload scanlations.
deserve to be treated right, just like every other artist.