囚人 (Shuujin)

Background
The song was first uploaded on August 11th 2008; a song entitled "Prisoner" with Kagamine Rin as the main vocal. It did not receive much attention, however, three months after the "original" version was uploaded to Nico Nico Douga, Len's version, along with a collab PV from Piapro featuring Nazekorewo-P, Madoromi, Carl, and more, was uploaded.

It became a hit with over 400,000 views, unlike Rin's version which failed to even reach 50,000 views, and instead sing it's "reply" song to Prisoner entitled "Paperplane". Shuujin-P put the tags "original song" in both the versions; so Len's version is not considered as a cover (same case scenario as Nori-P's Migikata no Cho).

Prisoner tells the tale of a boy in prison. One day he saw a girl who passes by the prison from time to time and decided to send a paperplane to her. Before long, they became friends and he fell in love with her. Unfortunately the girl had to "move away", leaving him in despair. The boy was later executed in a poison chamber, wondering what the girl's name was.

The song "Prisoner" later became one of the Append samples for "Power" (Len). Also, a novel adaptation entitled "Prisoner and Paperplane ~Boy Paradox~" was published on May 29th 2012, a new short Prisoner / Paperplane PV was also confirmed by Shuujin-P and was currently on the works.