Short answers, pulled from the story.
F.A.N.G is a playable character in Capcom's Street Fighter series, introduced in Street Fighter V in 2016 as one of sixteen launch characters. He is an assassin who fights using poison-based techniques and serves as the self-proclaimed second-in-command of the criminal organization Shadaloo, ranking below only M. Bison.
F.A.N.G is an acronym for "Fantastic Asian Notorious Gang," as revealed by the official Street Fighter social media account in February 2020.
F.A.N.G is consistently ranked near the bottom of Street Fighter V's tier lists because his charge-character mechanic requires players to hold directional inputs for a full second to execute moves, and post-launch patches from Capcom significantly reduced his damage output. His poison strategy also vanishes the moment he takes damage, limiting his offensive effectiveness in a game focused on offense.
F.A.N.G serves as the secondary antagonist in A Shadow Falls, acting as M. Bison's most loyal subordinate while opposing the alliance assembled by Karin Kanzuki and fending off the revived Charlie Nash and the entity Necalli. He is ultimately revealed to be the murderer of a friend of the fighter Rashid, who had uploaded a countermeasure program against Shadaloo before being killed.
The professional player Xian is the most prominent competitor associated with F.A.N.G. Known for mastering "unusual, unlikeable, and oft-misunderstood characters," Xian said playing F.A.N.G helped him understand the game at a deeper level, though he switched to Ibuki for professional competition as of 2017.
A.K.I. is a former protégé of F.A.N.G, originally introduced in the Street Fighter V side story Toxicity under the name Phantom. In Street Fighter 6, she appears as a downloadable playable character while F.A.N.G operates in World Tour mode as a non-playable character under the alias Fang Fei.