Aerith is a Mii from the Switch.
Aerith 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 (only if the player has purchased the Aerith DLC Mii Fighter costume as well as the Sephiroth DLC Fighter Pack), and she hasn't appeared in any game since. Her Mii data file filename is edge_spirits_mii01.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, and is one of few other playable characters.
Her Mii name in her Mii data file is エアリス1 (Aerith1).
Her Mii ID is as follows: ED 4F DD 86 56 79 46 33 8A C3 3C B0 D9 4F 1F 14
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 Aerith's Outfit does not display any changes if the Smash color is changed. So, at the moment, it can only be assumed her favorite color is the same as her Smash color, but currently, it's unknown for sure.
Aerith makes an appearance in one spirit battle, unlocked on the DLC Spirit Board if Fighters Pass 2 or Sephiroth's DLC is purchased, and if the Aerith's Outfit DLC Mii Fighter costume is purchased as well. If the Sephiroth DLC is not purchased, the player has no way to access the spirit battle, and if the Aerith's Outfit is not purchased, she will instead be replaced with a Zelda fighter. She will appear in the spirit battle for the spirit "Aerith" alongside Cloud. The Aerith Mii Fighter must be defeated for the player to win.
Her Smash color is currently unknown (as her outfit does not display it), and in battle, she always appears as a Swordfighter and wears the Aerith's Outfit with no headgear. Her neutral special is set to 1 - Gale Strike. Her side special is set to 3 - Chakram. Her up special is set to 1 - Stone Scabbard. Her down special is set to 2 - Reversal Slash. (Moveset: 1312.) Her voice is set to Type 6, with a Normal pitch. You can see a screenshot of how she appears in-game on the left.
Aerith 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 Aerith spirit, which can be seen on the left. She does appear as her Mii in an official screenshot, shown below.