Kokoro·Kiseki is one of the most popular and famous derivations of Kokoro. It serves as a prologue to the original song.
Kokoro·Kiseki tells the story of the scientist who created the robot in Kokoro. Shortly after creating the robot, the scientist realizes she is missing something important: her heart. Wishing to share happiness and sadness with her, he begins to work tirelessly at creating a heart for her. However, time is running out, for his life will eventually come to an end. As he begins to lose hope, a "miracle" suddenly happens: he receives a message from the robot from the future, after she has created her own heart. The scientist obtains peace and passes away in tranquility.
With Kokoro's official answer song that came two years later, it was revealed that Gackpo was the scientist but due to the popularity of the spin-off, many fanmade-PVs began to depict Len as the scientist after the release of Jun-P's work and still do.
This version sung by KAITO and MEIKO follows the same story as the original "Kokoro" and "Kokoro・Kiseki", with KAITO taking the role as the scientist and MEIKO portraying the robot. Both versions originally existed separately but were combined in the same fashion as Rin and Lens versions to form a harmonious melody when played together.