The term 'ONE OF THE MARIO BROTHERS' can be solved by the 5-letter word 'LUIGI'. Luigi is a fictional character featured in video games and related media released by Nintendo. He is the younger brother of Mario, the protagonist of the series. Luigi has appeared in every game of the main Mario franchise, and has also had cameos in various other games. He is portrayed as timid and cowardly, but is nonetheless loyal to his brother and willing to help him on his adventures.