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...
I can't tell where he is from inside here.
لا أستطيع تحديد مكانه من الدّاخل هنا
Von hier drinnen sehe ich nicht,
wo er sich aufhält.
Desde aquí no puedo saber
dónde se encuentra.
Desde aquí no puedo saber
dónde se encuentra.
Depuis le monde du miroir,
j’ai aucun moyen de le repérer.
Da qui dentro non posso sapere dov'è.
Daqui de dentro, eu não sei onde ele está...
Отсюда мне не понять, 
где он находится снаружи.