Deku Butler's Son

The Deku Butler's Son 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).

There is evidence to suggest that the Deku Butler's Son was killed before the start of the game, and that his soul 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 contain a soul. Also, 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 the fact that when Deku Link plays the song, a hollow shell of a Deku Scrub very similar to the Twisted Tree materializes, supports the idea that the Deku Mask possesses the soul of the Deku Butler's son.

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.