Kathrin is a Mii from the Wii.
Kathrin 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 052, however in Wii Music it is 10. Her Mii data filename is cpu_mii_052.rcd in Wii Sports, mii_052.rsd in Wii Sports Resort, Wii Fit, and Wii Fit Plus, mii_052.ffsd in Wii Fit U, and is cpu_mii_db10.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 has Mingling turned off. Her Mii name in her Mii data file is カトリン. In her Wii U/3DS-format Mii data file, she was originally created on a Wii (1).
Her Mii data file contains unused data. Her glasses are set to be red (Wii glasses color row 1 column 3), moved 3 down, and sized 1 smaller. Her mole is set to be moved horizontally 8 right, moved vertically 7 down, and sized 3 smaller.
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 80-4e-92-df and her console ID is c2-3d-21-af. Her Mii ID tells us she was created on August 27, 2006 at 9:33:48 AM.
According to a September 2020 Nintendo leak about CPU Miis, Kathrin's nationality is German.
In the files for Wii Sports, her dominant arm for Baseball (Batting) is right-handed, Baseball (Pitching) is right-handed, and Tennis is left-handed. Her unused dominant arm for Boxing is right-handed. Her Baseball Pitching style is normal. She's ranked 22 in Baseball, 20 in Boxing, and 12 in Tennis. Her group IDs are 2, 2, 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 left-handed. She's ranked 96 in Swordplay, 51 in Basketball, 8 in Table Tennis, and 88 in Cycling. Her unused Canoe and Debug ranks are both 52, matching her ID value of 052.
Sarah appears in promo art for Wii Party playing Globe Trot.
Names in other languages
Source: Wii Sports Resort