Warning: There are some spoilers, if you haven't finished the game, you have been warned. Not everything is 100% accurate.
Also, disclaimer: I do not own Deemo, Rayark does.
When he looked at the stump with a piano sitting atop it, something told him, he was dead.
When she fell down from the window high above on the ceiling, he caught her. He didn't know why, but he felt like she was connected to him in someway, something that he knew he wanted to get to the bottom of. Looking at her, she seemed so lonely, scared, but also so very determined to go back to her home. Some part of him wanted to help, as if it would help him find the answers he seeked. And something in him clicked. And in that moment, he realized something about all of the people here, and even so, he resolved to get the little girl out, because, it was a last chance to speak the words they never could before, "goodbye".
Each and every day he spent his time in the piano, playing and playing, and the little girl watched his performances with joy, smiling when he played. The sound of the piano filled the empty room with sound. And everytime he finished a song, the tree would sparkle, growing in height. Slowly but surely, they would reach the window someday.
When it stopped growing, they didn't know what was wrong. Something like that had never occurred before. It seemed so strange, and the girl saw the masked lady, walking happily around the tree, contrary to her previous bitterness. Even then, they desperately searched for a solution to this issue. When they discovered the underground room, he played the piano and the issue seemed to resolve itself, water now pouring in, as the tree grew and astonishing 10 meters.
Each day, the tree grew and grew. So much to see, yet so little too, knowing it was close to the end. A part of him wanted to keep her here, but it would be too cruel, she had her own wishes, besides, wasn't he dead already? The tree stump told him that a long time ago.
The closer they got to the end, the more he felt like stopping. When she left, this world would go with her, he knew that, something told him so. And when he reached the floating platform which would take her to the window, her tears started to spill but he continued on. There was no other choice, life was a fleeting thing.
And when the platform started to move towards the window, he could see that she remembered. He transformed back to his original form, slowing disappearing along with the masked lady and the world around him. And he knew he had one moment to say this, "Goodbye, my beloved sister Alice." And he knew he could rest at peace then, after all, never leave without saying goodbye.