Mii639 is a Mii from the Wii U.
Mii639 is an unused Mii found in the WaraWara Debug. When the Wii U originally shipped with system version 1.0.0, Nintendo accidentally left in old unused WaraWara Plaza files in a folder labeled "Debug". These were removed in the following system update. Many are in completely different formats than the one used by the final version of the WaraWara Plaza, but it's possible to still extract data from all of them. She, along with another 28 Miis, can be found in a file inside the Wii U NAND named follow_post.xml, with her Mii data file encoded in base64.
As with all WaraWara Debug Miis, she was originally made on a Wii U/Switch (4), her creator name is no name, has Copying turned on, Sharing turned on, the MAC is A4-C0-E1-F8-1B-FF, and unlike the WaraWara Plaza Miis is a valid Mii that can be saved to the Mii Maker. Her Mii name is Mii639 and her Mii ID is D0-00-02-7F. While it may seem possible to calculate a creation date from this, the time/date portion of the Mii ID is the number 639 in HEX, and since the Mii name has the same number, it's likely this is just used as a way to number the Mii rather than a proper time and date. Her height is set to 22 and her build is set to 94. While she does have unused nickname data, it is just junk data that can not be turned into any readable letters.
Her Mii data file contains unused data. Her mustache and her glasses are set to be moved down 2.
In the follow_post file, Mii639 will speak with a regular expression and has a painting. Her post has 8,787 replies.
The painting she uses is a common one seen among various other WaraWara Debug Miis with a painting.
Mii639 does not appear in any promo art.