The Friend originally appeared as the testing Mii used in the 3DS debug application FriendAddTool found in the September 2020 Nintendo leak. She hasn't appeared in any game. She can be found in the FriendAddTool.cpp file, with the hexadecimal of her Mii data file simply printed as text embedded in the code of the file.
She was originally made on a 3DS (3), her Mii name in her Mii data file is dummy girl, has Copying turned off, and Sharing turned on. Her console ID is D6-24-B7-96-5B-C6-2A-64, her Mii ID is 90-F0-E0-7A, and her MAC is 8C-56-C5-6B-47-5A. Her Mii ID tells us she was created on January 1st, 2011, 10:03:32 AM. Her height is set to 95 and her build is set to 40. Her birthday is set to January 1st, the same as her Mii ID creation date. She is set to be favorited.
Her Mii data file contains unused data. Her mouth color is set to the 3rd option. Her glasses are set to be sized 1 smaller and moved 1 up.
The Friend only appears in the debug 3DS application FriendAddTool. Once ran, the 3DS's Friend List would be filled with up to 100 copies of the Mii, all nicknamed "dummy girl", all permanently offline, and each given a non-working unique Friend Code. The status displayed for each friend is simply a number "1700000XXX", where XXX is replaced by a sequential number starting from 001, then 002, and so on, up to 100. A screenshot of the Friend List after running the FriendAddTool can be found in the image appearances section of Friend's page.
The name "Friend" was chosen to represent her on the Mii Library because of the purpose she serves in the FriendAddTool, where she becomes the user's friend in the Friend List as many times as possible.
It's interesting to see that here, the Friend uses the orange favorite color to represent the Friends List application, but the Friends List Miis of the WaraWara Plaza use the yellow favorite color to represent the same thing.
The Friend unfortunately does not appear in any promo art. Here is a screenshot of a 3DS's Friend List after running the FriendAddTool application:
Note that the number in this Friend's status, 003, does not match the friend index on the bottom screen, 12/100. This is because the 3DS had previous other friends added before running the FriendAddTool and the Friends got added to the end of the list. With a blank Friends List, the number in the status and the index on the bottom screen should always match.