Short answers, pulled from the story.
Sean Matsuda was created by the Capcom development team during the final days of developing Street Fighter III: New Generation. Producer Tomoshi Sadamoto made the decision to swap Ken Masters' head onto a new body to fill a roster slot before the deadline. Character designer Kinu Nishimura handled the visual design of the new fighter.
Sean Matsuda entered the world of Street Fighter III in 1997. He was created during the development of Street Fighter III: New Generation and later appeared in Street Fighter V as a supporting character for his sister Laura Matsuda. His image also appeared in Street Fighter 6 as cosmetic items.
Sean Matsuda is a young man of Brazilian and Japanese descent. He was born into a family connected to martial arts where his grandfather taught him jiu-jitsu and his uncles practiced capoeira. Academic analysis suggests his character symbolizes the Brasil Moleque culture and represents aspects of Brazilian life such as soccer and verticality.
Sean Matsuda was intended to be a handicap character designed for players to use against less skilled friends. His fighting style combines elements from Ken Masters and Ryu, including a headbutt that alludes to Vale Tudo. Critics noted that his moves are exaggerated and comical, often failing to pass qualifying rounds in local tournaments.
Sean Matsuda was initially voiced by Isshin Chiba in Street Fighter III. The role was later taken over by Mitsuo Iwata for his appearance in 3rd Strike. In Street Fighter V, he is voiced by Chris Rickabaugh in English and Ayumu Murase in Japanese.