Olivia Olson (the voice of Marceline) celebrated her 24th birthday on May 21, and as part of her celebration, Cartoon Network released Marceline's secret track, Happy Ending Song.
The song is from Marceline's bedroom recording sessions, based from her diary entries written about 500 years ago. Brace yourselves, the melody sounds VERY familiar.
Simple, yet so deep- just what I would expect from Marceline's compositions. I have a feeling that this song is dedicated to Simon/Ice King (again), but my gut says it's not. It sounds too much like a love song for it to be dedicated to Simon.