Lonesome Shadow

Luigi, the hero's brother, succeeds in saving Mario. However, in doing so, he disappears into a new world where Luigi is not the sidekick but the love interest! This new world is oddly more beautiful than his old world, and it is clear that Mario and Luigi do not exist in this perfect little world. King Boo has a strong desire to wed this strange human in any way possible, and Luigi doesn't seem to care until Bowser wants a part of their relationship. Then things get complicated. They eventually work things out, and suddenly Mario finds a way to this new world with Gadd, which causes major issues. Luigi has to deal with them to keep this world from dying and ends up making the ultimate decision to save this world he loves or save his brother.