You have to complete all 10 storylines in the game.
Chat with our AI personalities
Play the story constantly to get all 10 endings (when you have unlocked all the bosses) then you'll get last story.
Do the correct mission to play different levels in story mode. After 5 levels, there will be an ending. Play all levels to unlock the final boss.
Beat every stage and boss to reach the final story, and beat it. Devil Doom is the final boss.
You have to finish all of the stages in story mode.
To get Super Shadow in Shadow the Hedgehog (on any of the consoles it was released on), you must beat every ending (there are 10 of them) in the game. You only have to do one mission in each level (except the Stage Sixes). After that, you will unlock the last story. Play through the level and you will reach Black Doom. You will then turn to Super Shadow and fight Devil Doom. This is the only level where Super Shadow is playable.