Short answers, pulled from the story.
Noritaka Funamizu, the head of Capcom Research and Design, created William F. Guile to appeal to American fans and serve as a cultural bridge for Japanese developers. He debuted in 1991's Street Fighter II as one of the original eight World Warriors to resonate with the Western market.
Guile spent years believing his best friend and co-pilot Charlie Nash was dead after they were separated during a mission in Shadaloo. Charlie later died holding off M. Bison while Guile escaped, and this event haunts Guile through subsequent games including Street Fighter V where Charlie is resurrected.
The Sonic Boom is performed by holding a direction on the joystick and then pushing in the opposite direction with a punch to create a projectile. The Flash Kick is executed by holding a direction and then pushing in the opposite direction with a kick to launch Guile into the air.
Actor Jean-Claude Van Damme portrayed William F. Guile in the 1994 film Street Fighter and brought a physical presence that matched the character's military background. Van Damme was approached to reprise the role in the reboot Street Fighter: The Legend of Chun-Li but ultimately did not appear.
In Street Fighter V, Guile is promoted to a colonel rank and returns as one of the six DLC characters. He eventually decides to retire from fighting to be with his family and become a mentor to the son of late Robert Sullivan, Luke.
Guile ranked fourth in Gamest magazine's list of Best Characters introduced in 1991 and is often seen as the first image that comes to mind for Japanese audiences when thinking of an American soldier. University of Delaware professor Rachel Hutchinson discussed the use of imagery and military aspects to illustrate Guile as not Japanese and noted the increased use of American iconography in his presentation.