A song for Kagamine Rin. By Toraboruta-P (トラボルタP) w/ illustrations and PV by Fukusanbutsu-P (副産物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. - Wiki
Kokoro/Kiseki (Heart/Miracle) by Jun-P
A popular variation on the song, featuring Kagamine Len in the role of the professor. It tells the story of how he built her and came to make her future "heart", but he dies long before he can get to see her use it. The true answer song by Torabuta-P, "Sayonara, Arigatou", more or less follows the same plot, but has Kamui Gakupo in the role of the professor instead.