- Rumika Fukuda as Reina, a mysterious girl who dials the code "666" to transform into Kamen Rider Muse
- Daiji Asakawa (Hanjuku Famiria) as Jotaro Kikuchi, Keitaro's nephew who is the current owner of the dry cleaning shop where Takumi and friends live
- Rui Yanagawa as Hisao/Mosquito Orphnoch
- Ryunosuke Hashino (Izakaya Moheji) as Kota/Gekko Orphnoch
- Kanon Matsuzawa (Shojo☆Kageki Revue Starlight) as Kei/Quina Orphnoch
- Amane Shindo (BanG Dream!) as the new Smart Lady
Taking place 20 years after the events of the Kamen Rider 555 TV series, the film is written by Kamen Rider 555 series head writer Toshiki Inoue, and directed by Ryuta Tasaki (Kamen Rider 555: Paradise Lost), the film is set for a limited theatrical run this February 2, 2024 at the Shinjuku Wald 9 and other theaters, and will also get a Blu-ray/DVD release later.
[Additional info from Natalie]