The person hiding in the mirror was Illuso from the Hitman Team. Fugo ends up being dragged into the mirror by Illuso's Stand, Mirrorman. Fugo then summons his own Stand, Purple Smoke. Despite Giorno wanting to find Fugo, Abbacchio decides to go look for the key, since that was their mission. However, once Abbacchio finds the key, he also gets dragged into the mirror world...
Did you find what you 
were looking for? Well?
هل وجدتَ ما كنتَ تبحث عنه؟ هل وجدت؟
Gefunden, nach was du
gesucht hast? Sag schon!
¿Encontraste lo que buscabas? ¿Eh?
¿Encontraste lo que buscabas? ¿Eh?
Mon pote, t’as trouvé
ce que tu cherchais ?
Hai trovato ciò che cercavi? Eh?
Já encontrou o que estava procurando?
Где та вещица, которую ты ищешь, а?