Players looking to mess with the NPCs in The Legend of Zelda: Breath of the Wild will be interested to learn that the NPCs are made with a system that makes them surprisingly similar to Miis, the avatar characters introduced with the Wii. A modder explains that this means it's possible to take Miis and bring them into the game as NPCs, making them effectively Breath of the Wild Miis.
On Twitter, a Mii expert known as @HEYimheroic posted a series of tweets reporting that the NPCs in Breath of the Wild use an advanced form of the Mii creator tool. Because of this, she explains, it's possible to mod Miis to appear as NPCs in the game. In her example tweet, she shows off four NPCs and their respective Miis, including Matt from Wii Sports and Mii Gunner from Super Smash Bros. While there are a few minor differences, such as moles not being supported and some hairstyles not being supported, these "UMiis," as the game files refer to them as are effectively an evolution of Miis.
Hi, Mii expert here. Turns out, the NPCs in TLoZ:BotW use an advanced version of the Mii format. This means that with modding, you can inject Miis into the game. :)— i'm alice (@HEYimHeroic) January 4, 2021
Thinking about opening commissions for Mii injects, both screenshot/images of your Mii and mod downloads! pic.twitter.com/8NfVr4zyqA
In the follow-up tweets, HEYimheroic explains a few more common questions about UMiis and their associated characters. Important characters, like Link and Zelda, don't use UMiis and are instead pre-made 3D models. Every race of NPC uses UMiis, but only Hylians and Sheikah use Mii-like values; other races like Gorons and Zoras use different values. It's possible to backport UMii NPCs into Miis, but since they're more advanced, it's not a 1:1 conversion. Also, UMiis in Breath of the Wild isn't able to handle particularly outlandish Miis, with one comment citing RTGame's Mii "IWantDie."
HEYimheroic stated that there's still a lot about UMiis she has yet to uncover. She plans to upload more information on UMiis on her website "Mii Library" as it's uncovered, and also has plans to do Mii conversion commission in the future.