Sandy is a Mii from the Switch.
Sandy originally appeared as the default Mii for Neskdor town's Desert Celebrity role in the files of Miitopia (Switch, Demo), but unused within the demo itself. She later appeared in the full game of Miitopia (Switch). Once the player replaces her with a Mii of their choice, Sandy would never appear again, unless that Mii is deleted. Her Mii data file can be found in \cmn\storedata\npc_default_mii.sarc and is named Town01_Cereb.charinfo (a misspelling of "celeb", short for "celebrity").
Her Mii name in her Mii data file is せれぶk, which translates to "celeb k". Her height is set to 76 and her build is set to 83. Her Mii ID is as follows: 16 9C 72 F3 C5 00 40 45 B7 25 A5 08 62 9F 26 AE
Her Mii data file does not contain any unused Mii data.
Sandy's role in Miitopia (the Desert Celebrity) is described as so:
The names of the default Miis in Miitopia Switch often vary drastically depending on region. Sandy's name on the Mii Library is going off of the US English version, for consistency. A chart containing all of Sandy's names is located at the bottom of this page.
Sandy unfortunately does not appear in any promo art. Below is a screenshot of her in-game as her role in Miitopia.
This Mii originally appeared on the Switch!
Here's how to recreate them on the Switch:
Names in other languages
Source: Miitopia (Switch, Demo)