I said this won’t be a (Wa)Luigi blog, and I may or may not have been lying. Here’s a fanfic I wrote about the two. No romance (sorry??).
Fandom: Super Mario Bros/Luigi’s Mansion
Luigi learns how to be brave from arch-nemesis Waluigi, who took time out of his busy schedule to help Luigi with his little cowardice problem. How thoughtful! But not everything goes according to plan as unforeseen obstacles test the mettle of both teacher and student during the final “bravery lesson.”