If you're refering to Street Fighter IV, because 5 isn't out yet, basically, everyone has a different storyline but it pretty much revolves around Shadaloo.
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The storyline is a series of major shine sprites in the game, for example the first tutorial
You cant do storyline on 2p but if you go to two player there is stuff you can do there
There is no campaign for multiplayer. There is multiplayer which has no storyline, there is special ops where you can do co-op to do either a kind of campaign with a storyline with a friend or do survival mode to survive as many times as possible, or there is campaign mode, which is a single-player storyline.
You get it from Steven way later in the storyline
Talk to her after finishing the main storyline in the game.