Heihachi is a Mii from the Switch.
Heihachi appears as one of the Miis that are fought in the Kazuya DLC Spirit Board in Super Smash Bros. Ultimate since update 12.0.0, and he hasn't appeared in any game since. His Mii data file filename is demon_spirits_mii00.bin, with "demon" standing for Kazuya.
He is based off a character in Tekken with the same name. That character is the father of Kazuya, who is the fifth fighter in the Super Smash Bros. Ultimate Fighter's Pass 2.
His Mii name in his Mii data file is へいはち (Heihachi).
His Mii ID is as follows: 1B EA 7E BC F6 1B 4F 2A 88 ED 86 9A AA 08 03 4C
Currently, it's unknown whether or not he has unused Mii data - there is no way of knowing his Smash color (as Smash Ultimate can save a favorite color separate from a Mii's actual favorite color) as the Heihachi Outfit does not display any changes if the Smash color is changed. However, if the player has not bought the Heihachi Outfit DLC, then his Smash color is black, hiding his favorite color red. This could mean his Smash color with the Heihachi Outfit is also black, or even red, but there's no way to be sure.
Heihachi makes an appearance in one spirit battle, unlocked on the DLC Spirit Board if Fighters Pass 2 or Kazuya's DLC is purchased. He will appear in the spirit battle for the spirit "Heihachi Mishima". The Heihachi Mii Fighter must be defeated for the player to win. Depending on whether or not the player has bought the Heihachi Outfit DLC, his Smash information will change.
With the Heihachi Outfit Mii Fighter costume DLC (left), his Smash color is currently unknown (as his outfit does not display it), and in battle, he always appears as a Brawler and wears the Heihachi Wig and the Heihachi Outfit. His neutral special is set to 2 - Flashing Mach Punch. His side special is set to 1 - Onslaught. His up special is set to 1 - Lunar Launch. His down special is set to 3 - Counter Throw. (Moveset: 2113.) His voice is set to Type 9, with a Low pitch. You can see a screenshot of how he appears in-game (should the player have the Heihachi Outfit Mii Fighter costume DLC) on the left.
Without the Heihachi Outfit Mii Fighter costume DLC (right), his Smash color is black, and in battle, he always appears as a Brawler and wears the Dragon Helmet and the Fighter's Uniform. His neutral special is set to 2 - Flashing Mach Punch. His side special is set to 1 - Onslaught. His up special is set to 1 - Soaring Axe Kick. His down special is set to 3 - Counter Throw. (Moveset: 2113.) His voice is set to Type 9, with a Low pitch. You can see a screenshot of how he appears in-game (should the player not have the Heihachi Outfit Mii Fighter costume DLC) on the right.
Interestingly, Heihachi was one of the few Mii Fighter costumes from Super Smash Bros. for Wii U and 3DS with a unique Mii used in promo art that never had a QR code released (something most others had), making this the first time his Mii data file is available.
Heihachi has multiple appearances. In Super Smash Bros. Ultimate, his spirit and in-game appearance use his older design (left), while his Mii is based on his younger design (below), likely due to it being more iconic.
Heihachi appears as a Mii in Super Smash Bros. for Wii U and 3DS promo art, as the Heihachi Mii Fighter costume was also available in that entry. Below is that render.