Steph is a Mii from the Wii.
Steph originally appeared as one of the 60 CPU Miis to be included with Wii Sports. Since then, she's appeared in Wii Sports, Wii Sports Resort, Wii Party, Wii Music, Wii Fit, Wii Fit Plus, and Wii Fit U. In most of these games, her internal ID is 039, however in Wii Music it is 40. Her Mii data filename is cpu_mii_039.rcd in Wii Sports, mii_039.rsd in Wii Sports Resort, Wii Fit, and Wii Fit Plus, mii_039.ffsd in Wii Fit U, and is cpu_mii_db40.rsd in Wii Music. Wii Party combines all 100 Mii data files into one larger file named mii_rsd.bin in the same ordering as Wii Sports Resort.
As with every CPU Mii, she does not have a creator name or birthday, and has Mingling turned off. Her Mii name in her Mii data file is ステフ. In her Wii U/3DS-format Mii data file, she was originally created on a Wii (1).
Her Mii data file does not contain any unused data.
In Wii Music, her Mii data file is slightly different. While it's mostly the same as it is in Wii Sports Resort, it now includes a filled out Mii ID and console ID. Her Mii ID is 80-4f-8f-09 and her console ID is c2-3d-21-af. Her Mii ID tells us she was created on August 30, 2006 at 9:17:24 AM.
According to a September 2020 Nintendo leak about CPU Miis, Steph's nationality is North American.
In the files for Wii Sports, her dominant arm for Baseball (Batting) is right-handed, Baseball (Pitching) is right-handed, and Tennis is right-handed. Her unused dominant arm for Boxing is right-handed. Her Baseball Pitching style is normal. She's ranked 26 in Baseball, 49 in Boxing (meaning she doesn't appear in Boxing), and 15 in Tennis. Her group IDs are 9, 1, and 1.
In the files for Wii Sports Resort, her Wii country ID is 1, like all CPU Miis. Her dominant arm for Table Tennis is right-handed. She's ranked 58 in Swordplay, 79 in Basketball, 94 in Table Tennis, and 4 in Cycling. Her unused Canoe and Debug ranks are both 39, matching her ID value of 039.
Steph appears in promo art for Wii Music playing the bagpipes.
Names in other languages
Source: Wii Sports Resort