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...
And Fugo, Abbacchio, and I... 
will safely return to the others!
وأنا وفوغو وأباكّيو... سنعود بأمان
!إلى الآخرين
Und Fugo, Abbacchio und ich werden
es gemeinsam zurück zu den anderen schaffen!
Por último… ¡Fugo, Abbacchio y yo
regresaremos a salvo con el resto!
Y además… ¡Fugo, Abbacchio y yo
regresaremos a salvo con el resto!
Et je t’assure qu’on rentrera à trois,
Fugo, Abbacchio et moi !
E Fugo, Abbacchio e io... torneremo
tutti insieme sani e salvi dagli altri!
E Fugo, Abbacchio e eu... 
vamos voltar sãos e salvos!