Eiensei Riron

 (永遠性理論, Eiensei Riron) is sung by. The song lyrics are written by, and the song was composed by.

 was first released in the game as the first song of Part 4. If you clear Part 4 Chapter 1 it will be available to play within the normal lives. The full version of the song was released as part of the Re-raise Single on January 22, 2020. It can be purchased digitally on the iTunes Store, animelo mix, music.jp, mora, and Recochoku (レコチョク).

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