Tseng is a Mii from the Switch.
Tseng 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 he hasn't appeared in any game since. His Mii data file filename is edge_spirits_mii06.bin, with "edge" standing for Sephiroth.
He is based off a character in Final Fantasy VII with the same name. That character is allied with an investigative black-ops unit called the Turks, working for a company named Shinra. He shares this with Rude, Reno, and Elena (Switch).
His Mii name in his Mii data file is ツォン.
His Mii ID is as follows: 81 51 18 3B 86 B0 47 88 90 31 50 B5 7D CB DD 31
His Mii data file contains unused Mii data. In-game, his Smash color is set to white (as Smash Ultimate can save a favorite color separate from a Mii's actual favorite color), but his Mii data file has his favorite color set to red.
Tseng makes an appearance in one spirit battle, unlocked on the DLC Spirit Board if Fighters Pass 2 or Sephiroth's DLC is purchased. He will appear in the spirit battle for the spirit "The Turks & Rufus Shinra" alongside Reno and Rude. After a short period of time after the start of the battle, he will fall from the sky along with Elena (Switch). Once all four Mii Fighters have been defeated, Robin will appear holding the Steel Driver. He must be defeated for the player to win.
His Smash color is set to white, and in battle, he always appears as a Gunner and wears the Business Suit with no headgear. His neutral special is set to 2 - Laser Blaze. His side special is set to 2 - Stealth Burst. His up special is set to 2 - Cannon Jump Kick. His down special is set to 3 - Absorbing Vortex. (Moveset: 2223.) You can see a screenshot of how he appears in-game on the left.
Tseng unfortunately does not appear as a Mii in any promo art released. However, he does appear as his human character in promo art for Final Fantasy VII, which also appeared in Super Smash Bros. Ultimate as spirit art for The Turks & Rufus Shinra, which can be seen on the left. Tseng is the second from the left in the image.