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...
Ah, yes. You seem to be experiencing 
the same symptoms as me.
أجل. يبدو أنّك تعاني من نفس أعراضي
Ah, Tatsche. Du hast
dieselben Symptome wie ich.
Sí. Parece que tenemos
los mismos síntomas.
Sí. Parece que tenemos
los mismos síntomas.
Ah oui,
on a les mêmes symptômes.
Eh... Sembra proprio che tu stia
sperimentando gli stessi miei sintomi.
Ah, você deve estar sentindo
os mesmos sintomas que nós.
Да-а, симптомы у нас совпадают.