Mii644 is a Mii from the Wii U.
Mii644 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. He, along with another 28 Miis, can be found in a file inside the Wii U NAND named follow_post.xml, with his Mii data file encoded in base64.
As with all WaraWara Debug Miis, he was originally made on a Wii U/Switch (4), his creator name is no name, has Copying turned on, Sharing turned on, the MAC is A4-C0-E1-F8-1B-FF, and unlike the WaraWara Plaza Miis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. His Mii name is Mii644 and his Mii ID is D0-00-02-84. While it may seem possible to calculate a creation date from this, the time/date portion of the Mii ID is the number 644 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. His height is set to 73 and his build is set to 36. While he does have unused nickname data, it is just junk data that can not be turned into any readable letters.
His Mii data file contains unused data. His facial hair color is set to the 8th option. His mustache and his glasses are set to be moved down 2.
In the follow_post file, Mii644 will speak with a regular expression and says text. His post has 2,929 replies.
It should be noted that there is no game being referred to here. The icon this Mii stands under (along with every other follow_post Mii) is a generic DK Donkey Kong logo, not for any specific game.
Mii644 does not appear in any promo art.