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...
There's no way we'd comprehend this 
ability until we were actually attacked.
من المُستحيل أن نستوعب هذه القدرة
ما لم نتعرّض لهجوم فعليّاً
Natürlich hätten wir das kaum vorhersehen
können, bevor wir angegriffen wurden.
Sans être d’abord attaqué,
ton pouvoir reste une énigme.
È un potere impossibile da comprendere,
a meno che non se ne venga attaccati.
Não tínhamos como entender
este poder antes de sermos atacados.
Если честно, я не разобрался, 
что у тебя за сила, пока ты не напал.