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...
Once you've been infected, 
even I can't do anything about it.
حتّى أنا عاجزٌ عن فعل أي شيء
حالما تُصاب به
Ich kann nichts mehr tun,
sobald das geschehen ist!
No puedo hacer nada
en cuanto has sido infectado.
No puedo hacer nada
en cuanto has sido infectado.
Je ne suis pas capable
de soigner une contamination !
Una volta che qualcuno viene infettato,
neanche io posso farci più niente!
E nem eu posso te curar
dessa contaminação...
Я ничем не могу помочь заражённым!