SF2 is what lead to SF4 pretty much, Crapcom wanted to forget SF3 kind of.
SF3 didn't sell that well and it isn't casual friendly lol, like its the hardest SF game to get into. Crapcom wanted a new start so they kicked all the SF2 chars and made Alex the MC.
Ryu and Ken came back because they were irreplaceable, so they made little sub plots for them(ken has a family and ryu is just doing ryu stuff).
When they made SF4 they wanted it to cater to the casual crowd so they brought back literally all of the SF2 characters which is why SF4 is such a huge success. They kept supreme lord gentleman Dudley,Ibuki,Hugo,Yang and Yun but not Alex????
All the characters i liked got booted off for nostalgia SF2 fans to buy the game, its a good choice by Crapcom but still.
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Poison is female in Japan, she was a female in fatal fury but Murica didn't like men beating up women so they changed the poison looking enemies into male. Capcom said that she's a transgender to cover it all up, then the creator started making rumors and stuff like "its up to your imagination".
In SF x T Ryu and Chun li comment on how 'manly' poison is. But its all bs.
Its the same thing with Sheik and Zelda, can't have the beautiful princess of a well known game turn into a dude via black magic lol.