Okay, so story, uh

Her name's Princess Toadstool, and she was Mario's savior. One day, when she got kidnapped again, Mario had a made a different turn on accident, which is where he rescues Princess Peach instead, and the rest follows. Princess Toadstool was still in King Koopa's castle, watching her whole kingdom burn. She went insane at this rate, waiting and waiting for Mario; but he never came. 30 years has passed, and she was released from her cell, for whatever reason, at least. She wants revenge on him, for abandoning her for so many years, having to feel nothing but sorrow and rage. She has completely lost it at this point; making her: the Princess of Insanity.