Deku Mask

The Deku Mask is the first mask acquired in Majora's Mask. It is one of several masks that contains the spirit of another being; in this case, a Deku Scrub. Wearing the mask will cause Link to take on the form of the Deku Scrub, and gain new abilities. Unlike the Goron Mask and Zora Mask, the exact identity of the being whose body you inhabit while wearing this mask is unknown; however, there is much speculation as to just who it might be. (See The Spirit Within the Mask below.)

This mask is one of four Transformation Masks in the game, the other three being the Goron Mask, the Zora Mask, and the Fierce Deity's Mask.

Obtaining the Mask
The Deku Mask is a mandatory mask, and is also the first mask acquired in the game. Link acquires this mask upon learning the Song of Healing from the Happy Mask Salesman. In order to do this, Link must first retrieve the item that was stolen from him by the Skull Kid. The song causes the Deku spirit residing in Link to be encased within a mask, which then falls at his feet. The Happy Mask Salesman then tells Link to keep the mask as a memento.

The Spirit Within the Mask
During his travels through the inner Clock Tower, Link will run across a tree bearing a resemblance to a Deku Scrub, which Tatl even points out as looking like Link. During the ending credits cutscene, the Deku Butler (who lost his son ) is seen standing near the tree, implying that the tree is his son, most likely killed by Skull Kid, who then used that spirit to transform Link. 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.