As you make your way through Bowser's Castle, you reach the room where you would normally fight Bowser Himself, but instead, standing there was... Luigi? wait, why is Luigi here?

He had his back turned, facing away from you. "Just about time. You know Mario, I always felt like I only stood in your shadow. Alone, forgotten, with you always leaving me behind for your precious princess. Everyone loves Mario, everyone overlooks his brother. And yet, I've endured it all. You are my brother, I'll upset me if I hurt you. I don't feel disposed to do this. But somehow. My brother's enemy offered me a better life, than a life living with you. It's ironic. I am not wasting my lifetime..." He spins around in a fighting stance. "You thought Koopa worked alo-" He suddenly realizes something; you weren't Mario. "-...eh? You are not Mario..." (Who are you?)