Short answers, pulled from the story.
F.A.N.G is the self-proclaimed second-in-command of the villainous organization Shadaloo, ranking only below the infamous M. Bison. He is a tall, spindly assassin who uses poison to complement his fighting style and serves as the story's overall secondary antagonist in the mode A Shadow Falls.
The character was originally considered during the development of Street Fighter III: 3rd Strike, but the idea was shelved until Street Fighter V launched in 2016. He appears as a non-playable character in Street Fighter 6's World Tour Mode where he goes under the pseudonym Fang Fei.
F.A.N.G was originally the leader of the Nguuhao Cartel, a criminal organization that attempted to pledge false allegiance to Shadaloo in a plot to murder M. Bison. The scheme backfired catastrophically when Bison slaughtered the entire cartel, leaving F.A.N.G as the sole survivor who then swore unyielding loyalty to Bison.
F.A.N.G is a charge character requiring second-long maintained button presses in various directions to properly execute moves. His primary tactic involves keeping the opponent perpetually poisoned to wear them down and make opportunistic attacks using his long reach.
F.A.N.G was placed 78th in a worldwide Street Fighter character poll held between 2017 and 2018, consistently ranking as the least used character. Patch updates following the game's launch have greatly reduced the character's damage output, which spells trouble in a game so focused on offense.