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...
So, what? Can it only 
defeat powerless humans?
ماذا إذاً؟ لا يستطيع هزيمة أحد غير البشر الضُعفاء
Kommt der nur gegen Leute an,
die sich nicht wehren können?
¿Solo puede contra humanos indefensos?
¿Solo puede contra humanos indefensos?
Tu ne peux vaincre
que des hommes impuissants ?
Che c'è, funziona solo
contro le persone indifese?
Quer dizer que você só
consegue derrotar humanos indefesos?
Только и можешь, что бить
беззащитных людей, да?