d´après gamefaqs
You´re in a room with mirrors in it. Notice Lakitu, the cameraman, in the
mirror. Walk up to it and Mario´s reflection becomes that of Luigi. Talk
about insulting! Big Boo appears, wearing a crown ( which leads me to believe
that Big Boo is also King Boo). He locked away the wimpy green mustache man,
and he will do the same to you. Big Boo sticks inside the mirror, rarely
coming out. Ground pound his shadow, which can attack you, on the non-mirror
side. When Big Boo fails after three hits, he takes off. Take Luigi´s Key.