"Kokoro" is a song featuring Kagamine Rin by Toraboruta-P.
The song tells a story about a robot (Rin) made by a lonely scientist, but is missing a program unable to be made: a heart. Some time after her creation, her scientist passes away, leaving her alone for several hundred years. She begins to wonder about her creation and makes a wish. She accesses the files that her creator had left and finally gains a heart. In the process, she discovers what it's like to be happy and sad. With her new feelings, she gratefully dedicates all of the songs that she sings to her creator. In the end, the miracle robot had a short in her system and could no longer sing. However, she had a smile on her face in her last moments, resembling an angel.
A prequel named Goodbye, Thank You was made for the album of the same name. It tells the story of the robot's creation and the making of the heart told from the scientist's (Gackpo) point of view.
This song has entered the Hall of Legend.