Deku Butler's Son

The is a character mentioned by his father, the Deku Butler, in Majora's Mask. After leading Link to the Mask of Scents, the Deku Butler apologizes to Link for traveling so fast, stating that Link reminds him of his son, whom he used to race through the labyrinth.

Previously, during his travels through the inner Clock Tower, Link ran across a tree bearing a resemblance to a Deku Scrub. When Z-targeting the twisted tree at the start of the game, Tatl notes that the tree looks a bit like Deku Link. During the extended end credits cutscene, the Deku Butler is seen mourning near the twisted tree, implying that the tree is his son, most likely killed (possibly by Skull Kid possessed by Majora's Mask, who then used that spirit to transform Link).

The soul of the Deku Butler's Son is within Link's Deku Mask: the other two main transformational masks, the Goron Mask and the Zora Mask, contain the souls of the deceased Darmani and Mikau respectively, suggesting that the Deku Mask does indeed also contain a soul. Moreover, the Song of Healing was required to create each of the masks. The empty "shells" produced by the "Elegy of Emptiness" when played as Zora Link or Goron Link appear identical, if not strikingly similar to Mikau and Darmani, suggesting that the song pulls the hollowed form from the soul of the user. This observation, coupled with Deku Link's hollow shell resembling an untwisted version of the tree, supports the idea that the Deku Mask possesses the soul of the Deku Butler's son.

The book Hyrule Historia reconfirms this theory a decade after the release of Majora's Mask.

Trivia

 * In the Majora's Mask manga, it is said that the Butler's son looked exactly like Deku Link, and we find out that the Deku Princess was in love with him.
 * The Flute Boy from A Link to the Past also takes the shape of a small, gnarled tree when he passes away.