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...
Shattering the mirror didn't 
actually work in your favor!
إنّ تكسير المرآة لم يكن من صالحِك
!في واقعِ الأمر
Es hat dir gar nichts gebracht,
dass der Spiegel in Scherben liegt.
No te hice un favor al romper el espejo.
No te hice un favor al romper el espejo.
Ton histoire d’effet inverse,
c’est du pipeau.
Rompendo lo specchio
non ti ho fatto nessun favore.
O espelho estilhaçado não te favoreceu!
Разбитое зеркало сыграло на руку мне!