Who is the Deku in the Deku Mask?

In Majora's mask, the masks of Transformation are all created from "tormented souls", that Link uses the song of healing on, and they are then transformed into masks. I.e. the Zora Mask is created from Mikau, and the Goron Mask is created from the Ghost of Darmani III.

These both have their own dedicated mini-stories to them, establishing them as real characters in the story of the game, whereas when the Skull Kid transforms Link into a Deku, it comes across as more of a curse than binding him to the soul of another. And again, once he plays the Song of Healing for the first time, it simply shows Link waving at the Deku as it retreats, and the Deku mask drops at his feet.

Is this Deku significant in anyway, or even a real character, or is this simply a curse created by the Skull Kid?

Solution 1:

While never explicitly stated, the end credits for Majora's Mask shows the Deku Butler mourning at a tree found near the very start of the game. The assumption being that it's his missing son that he mentioned during the game.

Solution 2:

As @Powerlord stated, while it is never explicitly stated, it is heavily implied that Link's Deku mask comes from the tree he encounters at the very beginning of the game.

As soon as Link encounters it, which is shortly after he turned into a Deku, Tatl (the fairy) remarks that he "sort of looks like this tree..."

Later in the game, after saving the Deku princess, Link can challenge the Deku Butler to a race. During the mini-game, the Deku Butler mentions how Link reminds him of his son:

And finally, after the game ends, we see the Deku Butler crying in front of that same tree:

According to Zelda Dungeon, this post game scene is only shown if you received the Mask of Scents for completing the Deku Butler's mini-game.

During the Deku Butler's mini-game, he mentions that Link reminds him of his son. If Link completes the mini-game and is awarded the Mask of Scents, Link will also be rewarded with a special cutscene prior to the game's credits. This special scene features the Deku Butler sobbing at the foot of the Deku-like tree, seen near the beginning of the game, at which time Tatl remarked that the tree seems to have a look of pain.