The original version (sung by Len, not the original composer) of the song "Let me Play the Piano" was removed from Niconico on April 26, 2011. The rearrange version and the original version were removed permanently from Niconico, and the "Append version" sung by Len Append Serious was uploaded as a "replacement".
The song starts with Len saying he'll never tell Rin he loves her; he doesn't want her to know, yet he wants to play the piano for her. As the song goes on, it tells that Len saw Rin with a man, and she gave him a warm smile (in the original PV that was taken down by Hinata Haruhana, the man is KAITO).
Len feels jealous, though he doesn't want to be selfish. He chooses to let Rin be happy with the man, and leaves them alone.