Gunner is a Mii from the Switch.
Gunner originally appeared as the default Mii for the Mii Gunner fighter in Super Smash Bros. Ultimate. She hasn't appeared in any game since. Her Mii data file filename is mii_gunner.bin.
Her Mii name in her Mii data file is しゃげきJ, which translates to "Shageki J". "Shageki" means shooting. While the "J" part is currently unknown, this makes Gunner's name "Shageki J". Despite this, the name on the Mii Library is still her title ("Gunner") as this is what she is called in-game and this is information is only known through unused Mii data file info.
Her Mii ID is as follows: 8F 61 6D D7 F0 14 4A CB A1 52 68 44 7E 85 FB 9F
As she is a Smash Bros. character, she has very high quality promo art of her character render, as seen on the left. In Super Smash Bros. Ultimate, she is considered fighter #53.
She is one of few Miis that have a voice actor. In Super Smash Bros. Ultimate, Mii Gunner can choose from 12 different voices, all voiced by Yūji Kishi, Takashi Ōhara, Ryōtarō Okiayu, Michihiko Hagi, Hideo Ishikawa, Kiyoyuki Yanada, Umeka Shōji, Ayumi Fujimura, Makiko Ōmoto, Minami Takayama, and Kimiko Saitō.
Outside of the above Smash Bros. character render, Gunner unfortunately does not appear in much else promo art, aside from the large murals featuring all the characters in the game. There are some promotional screenshots of the Mii Gunner, however, as shown here. For some images, you may have to open them in a new tab to see the whole render.