The official website of Lonely Castle In The Mirror anime film announced on Nov 6, 2022, that Shingo Fujimori and Karen Takizawa have been cast as voice actors in the film. Shingo will voice Kokoro’s homeroom teacher, Ida-sensei, while Karen will voice the school nurse.

The site also released the actor comments, which are as follows:

Shingo Fujimori comment:

I was nervous because it is a very famous work and I had to be a part of this worldview, so I felt a lot of pressure on myself.
Mr. Ida is a good teacher with a refreshing image at first glance, but he seems to be frivolous, saying nice things but without substance. I feel that I was cast in the film because of this similarity.
This is a work that fans of the original work and those who have not read the original work can enjoy and think deeply about. Please come to the theater and enjoy this wonderful work with your loved ones.

Karen Takizawa comment:

I was very nervous during the dubbing, but I enjoyed it very much because I love performing. I loved Director Hara’s work, but I had never met him before, so I did quite a bit of image searching (of his face) to see what he looked like… I was happy to be taught by him so kindly.
I am happy to have been invited to perform in this film, and I will go to the theater to see it with the same feelings as the audience.
With the power of animation to create a fantastical worldview, no matter how excited you are about this work, it will exceed your excitement, so please come and see it.

It was previously announced that 15-year old voice actress Ami Touma has been cast to voice the protagonist Kokoro. 

Other cast members include:

• Sakura Kiryu as Aki
• Takumi Kitamura as Rion
• Aoi Miyazaki as Kitajima-sensei
• Rihito Itagaki as Subaru
• Naho Yokomizo as Fūka
• Minami Takayama
• Yuki Kaji as Ureshino
• Kumiko Aso as Kokoro’s mother
• Yuki Kaji as Ureshino
• Mana Ashida as Okami-sama

The staff involved in Lonely Castle In The Mirror includes:

• Director: Keiichi Hara
• Screenplay: Miho Maruo
• Music: Harumi Fuuki
• Character Design: Keigo Sasaki
• Chief Animation Director: Keigo Sasaki
• Lonely Castle Design: Ilya Kuvshinov
• Visual Concept: Ilya Kuvshinov

A-1 Pictures is in charge of the film’s animation. The film will premiere in Japan on Dec 23, 2022.

Lonely Castle in the Mirror is a Japanese novel written by Mizuki Tsujimura, and published by Poplar Publishing in May 2017. A manga adaptation illustrated by Tomo Taketomi was serialized in Shueisha’s seinen manga magazine Ultra Jump from June 2019 to February 2022, with its chapters collected into five tankōbon volumes.

The official website describes the plot of Lonely Castle In The Mirror as:

One day, a mirror in Kokoro’s room suddenly began to glow in front of her eyes, as she had lost her place at school and was confined to her room. She walked through the shining mirror to find a mysterious castle-like building. There were seven people in a similar situation to Kokoro. It is said that a secret “key” is hidden in the castle, and whoever finds it will have their wish come true. Why were these seven people gathered together? and where is the key? When everything is revealed, you will be surprised and moved.

Source: Official Website