Shadow Mario is.. well, do you really want to know the secret?.. okay, I'll tell you. He's secretly Bowser. JR, but he got an invite to SMSC for "Shadow Mario", not Bowser JR, he was planning on going in without, but he thought it'd be fun.. now he's first out and stuck in the Minus World. Other than that, Shadow Mario did cause some trouble in an island, but the real Mario came to stop him. Overall, he's just Bowser JR..