Jace is a Mii from the Switch.
Jace originally appeared as the default Mii for the Prince from a Nearby Land role in the files of Miitopia (Switch, Demo), but unused within the demo itself. He later appeared in the full game of Miitopia (Switch). Once the player replaces him with a Mii of their choice, Jace would never appear again, unless that Mii is deleted. His Mii data file can be found in \cmn\storedata\npc_default_mii.sarc and is named Castle00_Prince00.charinfo.
His Mii name in his Mii data file is となりおうじk, which translates to "neighboring man k". His height is set to 0 (minimum) and his build is set to 127 (maximum). His Mii ID is as follows: 78 A7 E1 1F C7 09 40 73 8F 84 3F C5 E9 03 55 4A
His Mii data file does not contain any unused Mii data.
Jace's role in Miitopia (the Prince from a Nearby Land) is described as so:
He seems to bear resemblance to a Mii featured in Miitopia (3DS) promo art, as it seems a handful of the Miitopia Switch default Miis were based around Miis that appeared in promo art for the original game, but it's unconfirmed if Jace and the promo art Mii are meant to be the same character.
The names of the default Miis in Miitopia Switch often vary drastically depending on region. Jace's name on the Mii Library is going off of the US English version, for consistency. A chart containing all of Jace's names is located at the bottom of this page.
Jace unfortunately does not appear in any promo art. Below is a screenshot of him in-game as his 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)