it doesnt have to be official, but wholistically, if a character is featured prominently on the box, has a play style that is broad enough to be appealing to lots of players, and has a narrative that represents the essence of the fighting competition the game is based around, that is probably "the" or "a" MC.
according to my own criteria:
ryu is the MC for SF1 and SF2. the SF2 tagline "the world warrior" is probably best exemplified by ryu since his motivations are purely about traveling the world to fight the best. he wears a simple white gi, which in a Japanese context, is about as pure martial arts as it gets.
he often is featured alone or is the focus when on marketing materials with other characters during the SF1 and 2 era.
his fighting style allows for both up close striking and long range projectile play.
he stopped being the MC for each game after that, but he still is the MC for the series as a whole or even all fighting game series since his quest to be the best through world competition is essentially what the game players are doing when playing locally or online against another human opponent.