Tifa is a Mii from the Switch.
Tifa appears as one of the Miis that are fought in the Sephiroth DLC Spirit Board in Super Smash Bros. Ultimate since update 10.1.0, and she hasn't appeared in any game since. Her Mii data file filename is edge_spirits_mii00.bin, with "edge" standing for Sephiroth.
She is based off a character in Final Fantasy VII with the same name. That character is allied with the main character. She's also allied with a group called the Avalanche, and she shares this with Jessie (Switch), Wedge, Biggs, and Barret.
Her Mii name in her Mii data file is ティファ1 (Tifa1).
Her Mii ID is as follows: 8E CF 58 F5 5E 8E 47 43 8A 9F E5 4D 79 E4 BE 76
Currently, it's unknown whether or not she has unused Mii data - there is no way of knowing her Smash color (as Smash Ultimate can save a favorite color separate from a Mii's actual favorite color) as the Tifa's Outfit does not display any changes if the Smash color is changed. However, if the player has not bought the Tifa's Outfit DLC, then her Smash color is black, hiding her favorite color red. This could mean her Smash color with the Tifa's Outfit is also black, but there's no way to be sure.
Tifa makes an appearance in one spirit battle, unlocked on the DLC Spirit Board if Fighters Pass 2 or Sephiroth's DLC is purchased. She will appear in the spirit battle for the spirit "Tifa" alone. The Tifa Mii Fighter must be defeated for the player to win. Depending on whether or not the player has bought the Tifa's Outfit DLC, her Smash information will change.
With the Tifa's Outfit Mii Fighter costume DLC (left), her Smash color is currently unknown (as her outfit does not display it), and in battle, she always appears as a Brawler and wears the Tifa's Outfit with no headgear. Her neutral special is set to 2 - Flashing Mach Punch. Her side special is set to 1 - Onslaught. Her up special is set to 3 - Thrust Uppercut. Her down special is set to 2 - Feint Jump. (Moveset: 2132.) Her voice is set to Type 2, with a High pitch. You can see a screenshot of how she appears in-game (should the player have the Tifa's Outfit Mii Fighter costume DLC) on the left.
Without the Tifa's Outfit Mii Fighter costume DLC (right), her Smash color is black, and in battle, she always appears as a Brawler and wears the Protective Gear with no headgear. Her neutral special is set to 2 - Flashing Mach Punch. Her side special is set to 1 - Onslaught. Her up special is set to 1 - Soaring Axe Kick. Her down special is set to 2 - Feint Jump. (Moveset: 2112.) Her voice is set to Type 2, with a High pitch. You can see a screenshot of how she appears in-game (should the player not have the Tifa's Outfit Mii Fighter costume DLC) on the right.
Tifa unfortunately does not appear as a Mii in any promo art released. However, she does appear as her human character in promo art for Final Fantasy VII, which also appeared in Super Smash Bros. Ultimate as spirit art for the Tifa spirit, which can be seen on the left. She does appear as her Mii in an official screenshot, shown below.