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...
You couldn't see who you 
dragged in here, could you?
لم تستطع رؤية هويّة الشّخص الذي
سحبتَه إلى هنا، أليس كذلك؟
Du hast gar nicht mehr sehen können,
wen du zu dir gezogen hast, oder?
No viste bien
a quién arrastraste, ¿verdad?
No viste bien
a quién arrastraste, ¿verdad?
Tu t’en es pris à la mauvaise cible.
Non sei riuscito a vedere bene
chi stavi facendo entrare, vero?
Você não viu quem você
arrastou pro seu mundo, né?
Ты даже не заметил, 
кого схватил на самом деле!