In a
Q&A interview for the
upcoming Ready Player One film,
Ernest Cline, who wrote the original novel and the screenplay for the film confirmed that
Ultraman will not be appearing in the film.
Prior to the unveiling of the trailer, he revealed that "We weren't able to get everything we wanted, but we got the mass majority of it. There are some differences between the movie and the book, but I was involved with all those changes."
After the trailer played, around the 19:30 mark of the Q&A, Cline revealed that they replaced Ultraman with the Iron Giant because they cannot get the rights to use Ultraman in the film. It is because of a legal battle between Tsuburaya Productions and "another company from Japan" presumably, UM Corporation.
Both companies want Ultraman to be in the film, "but the rights could not be cleared because of their legal battle over it", causing them to find a replacement for the Giant of Light. As for said legal battle,
Tsuburaya won the case. The film's director, Steven Spielberg suggested to use Iron Giant as a replacement for Ultraman for the film, which he is okay with as he is also a fan of the Iron Giant.