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...
But I'm not letting that 
brat named Giorno get away!
لكن لن أسمح لذلك الطفل الشقيّ المدعو
!جورنو بالهرب
Aber dieser Dreikäsehoch
Giorno entkommt mir nicht.
Pero no dejaré
que el mocoso de Giorno se escape.
Pero no dejaré
que el mocoso de Giorno se escape.
Je laisserai pas le gamin
prendre la fuite !
Ma non lascerò che quel moccioso
di Giorno se ne vada via così!
Mas aquele moleque do Giorno não me escapa!
Только щенок Джорно никуда не денется!