ぼくにピアノを弾かせて (Boku ni Piano wo Hikasete)

Background
The original version (sung by Len; not the original composer) of the song "Let me Play the Piano" was removed from Nico Nico Douga on April 26th 2011. At the time, the rearrange version and also the original version was removed permanently from Nico Nico Douga, the "Append version" sung by Len Append Serious was uploaded as a "replacement".

The song starts with Len who said he'll never tell Rin he loves her, he doesn't want her to know, yet he wants him to play the piano for her. As the song goes, it tells that Len saw Rin with a man, and she gave her a warm smile (in the original PV that was taken down by Hinata Haruhana, the man is Kaito).

He felt jealous, though he didn't want to be selfish. He choose for Rin to be happy with that person, leaving him alone.