Short answers, pulled from the story.
Producer Tomoshi Sadamoto created Gill for Street Fighter III: New Generation in 1997. He envisioned a multifaceted science fiction character who could control fire and ice to contrast with previous antagonists.
Gill stands 6 ft 10 in tall as a muscular man with long flowing blond hair. The right side of his body is colored red while the left side is colored blue, and this dual coloring remains consistent regardless of which direction he faces during gameplay.
Gill serves as the President of the Illuminati secret society that has controlled the underworld for thousands of years. His goal is to turn the whole world into a utopia by causing an armageddon through manipulation of fire and ice.
Gill made his playable debut in Street Fighter V as part of the Champion Edition update. Before this moment he was not selectable in any arcade versions of the Street Fighter III games though players could unlock him from console versions of 2nd Impact and 3rd Strike only after meeting specific conditions.
Urien is Gill's younger brother who appears in 2nd Impact as a player character with similar build and attire. During the course of the story Gill is revealed to have been promoted after Urien takes over his former presidency within the Illuminati organization.