Deemo is a mysterious being who lives in solitude in a castle, all by themselves. A little girl falls from the sky, not knowing who she is, where she comes from. To help the little girl back to her world, Deemo plays their piano to grow the tree atop of it to reach the top of the tower. As the player continues to grow the tree through playing the game, the storyline gradually unravels...

“Never left without saying goodbye.” 
 
A girl who lost her way and found herself in that mysterious place met “Deemo”, a man dressed completely in black. There, every time they played the piano a tree would grow taller, leading them to the “place” where the girl would have to return. And then, the “dream” that both the girl and Deemo wished for at the end is― 
 
(Source: Deemo Wikia)

A girl who fell from the skies and lost her past Deemo who plays the piano all alone in the world of the treehouse an accidental encounter between the two. The music flows as the fingers hit the piano keys. The start of a fairytale journey has begun... 
 
'Before turning to leave, don't forget to say one last goodbye.' 
 
(Source: Anime News Network)

Manga adaptation of DEEMO: Memorial Keys.