It seems like The Legend of Zelda: Breath of the Wild uses an upgraded version of Nintendo’s classic Mii character creator to design many of Nintendo’s NPCs.
Inside the Breath the Wild files, NPCs are referred to as “UMii” (Ultimate Miis?) That use a combination of familiar Mii face components for their various designs.
Versions of these components from Breath of the Wild are more detailed and stylized to match the game’s art style. However, it is noteworthy how many NPC characters share various bits and pieces of the same facepieces.
This also means that with a little bit of code injection, customizing the game’s UMii characters to display with different face parts, and even your Mii in Breath of the Wild, as HEYimHeroic demonstrates, can also be achieved:
UMiis represents most of the NPCs from both the human and hylian races of the game. Unfortunately, there is no way you can turn yourself into a goron.
Also, you cannot use this method to edit the designs of the main characters whose character models are created separately. So you will have to dress or strip Link as standard.