"Prisoner / Paperplane" (囚人紙飛行機) is originally a Len original song (Prisoner) and an answer song (Paperplane) first uploaded on 2008/2009 period. It later became notable enough to receive a background series, a novel series, and also a manga adaptation by Akamura Aki. Though the song, novel, and manga may have different plots and also additional characters (in it's background series on the Prisoner Series, the girl's father is present) it all revolves on two "lovers", a prisoner and a sick person in which both of them died in their own respective point of views.