Short answers, pulled from the story.
Gill is a character in Capcom's Street Fighter series, first appearing in Street Fighter III: New Generation. He serves as the main antagonist and final boss of the Street Fighter III series, holding the presidency of a secret society called the Illuminati.
Gill was developed under producer Tomoshi Sadamoto, with character design by Akira Yasuda, known professionally as Akiman. Yasuda's early rough draft featured a line drawn down the middle of the character, separating his body into two colors, which became Gill's defining visual trait.
The split design originated with character designer Akira Yasuda's rough draft, which Sadamoto associated with Android Kikaider and J. Geil from JoJo's Bizarre Adventure. In gameplay, Gill's red and blue sides always appear on the correct sides regardless of which direction he faces, and his abilities change based on his facing direction.
Gill is not playable in the arcade versions of any Street Fighter III game. He can be unlocked as a playable character in the console versions of Street Fighter III: 2nd Impact and 3rd Strike. His first appearance as a fully playable character in a non-console release came in Street Fighter V's Champion Edition update.
Gill received mixed reception. Paste magazine called him polarizing and a one-note character. GamesRadar+'s Lucas Sullivan called him the godfather of infuriating final boss fights, citing his resurrection ability as the mechanic that most fundamentally changed how players approached the encounter. Den of Geek's Gavin Jasper considered the character a genuine success compared to M. Bison.
Gill's resurrection ability allows him to revive himself during a fight, which Lucas Sullivan of GamesRadar+ described as fundamentally changing how players approached the encounter. Rather than simply depleting his health, players had to find ways to prevent the resurrection before landing the finishing blow.