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...
As for your Stand, 
I permit half of it to get out!
وبالنسبة للستاند الخاص بك، سأسمح
!لنصفِه بالخروج
Deinem Stand erlaube ich dafür,
zur Hälfte hinauszugehen.
¡Y expulsaré
la mitad del cuerpo de tu Stand!
¡Y expulsaré
la mitad del cuerpo de tu Stand!
Per quanto riguarda il tuo Stand invece,
darò il permesso a una sua metà di uscire di qui!
E permitir que apenas metade
do seu Stand saia!