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...
Of course we couldn't know.
من الطبيعي ألّا نعلم
Woher sollen wir das auch wissen?
Con algo así, no teníamos forma
de saber cómo nos atacarían.
Con algo así, era imposible
saber cómo nos atacarían.
On risquait pas de piger.
Era ovvio che non capissimo...
Não tínhamos como prever, né?
Конечно, мы не поняли.