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...
The one who truly risked his 
life here... was Abbacchio.
الشّخص الذي خاطرَ بحياته فعليّاً هنا
كان أباكّيو...
Wer hier wirklich sein Leben
riskiert hat, das war Abbacchio.
Quien de verdad puso su vida en juego
fue Abbacchio.
Quien de verdad puso su vida en juego
fue Abbacchio.
Quello che ha davvero rischiato
la sua vita... è stato Abbacchio.
Quem arriscou a própria vida...
foi o Abbacchio.