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 part of me that's infected 
is not allowed to leave!
!الجزء المُصاب منّي لا يُسمَح له بالمُغادرة
Dem Teil von mir, der infiziert ist,
ist es untersagt, den Spiegel zu verlassen!
¡La parte infectada de mi cuerpo
no tiene permitido salir!
¡La parte infectada de mi cuerpo
no tiene permitido salir!
La partie contaminée de mon corps
n’a pas le droit de partir !
Non dare alla parte infetta
del mio corpo il permesso di andarsene!
Deixe a parte infectada aqui dentro!
Заражённые клетки со мной не выпускать!