17 U.S. Code § 107 - Limitations on exclusive rights: Fair use
Notwithstanding the provisions of sections 106 and 106A, the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—
Japanese View on Fanmade Comics (Doujins)
In Japan, doujins are not seen as illegal or disrespectful. They are used as another form of free marketing. Mangakas respect the amateur artists (1), want to maintain a good fan base (2), and they’re considered parody (3) (https://www.tofugu.com/japan/doujinshi-definition/).
Shinzo Abe the prime minister of JAPAN, saying they should not be viewed as theft, but instead as parody:As it is, the doujinshi business wouldn't fly outside of Japan. Other countries' more militant copyright laws would not tolerate the sale of fan-comics based on copyrighted property -- selling a fan-work of your favorite Marvel Comics character, for instance, isn't safe if you don't want to be sued. Luckily, the Japanese see it differently; instead of viewing doujinshi as a threat to original works, they're generally considered healthy, promotional and celebratory. Recently, Shinzo Abe (the literal Prime Minister of Japan) came out in support. Defending doujinshi from the copyright laws of the Trans-Pacific Partnership, he argued that doujinshi do not exist in the same market as the works they take from, and they should be viewed less as theft and more as parody. Doujinshi is also aided by the fact that, culturally, the Japanese are less likely to litigate.
AOT no Requiem will never be for profit, however in Japan an event called Comiket exists where doujins are sold FOR PROFIT:Comiket now attracts roughly one million visitors yearly, with nine million doujinshi sold per market. Meiji University’s Yoshihiro Yonezawa Memorial Library is entirely dedicated to the collection and preservation of hundreds of thousands of fancomics – their status elevated to that of high cultural and historical worth, worthy of their own archive.