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...
If I can get your reflection 
with a different shard,
إذا استطعتُ الحصول على انعكاسِك
،بقطعةٍ مختلِفة
Wenn ich deine Reflexion
auf einer anderen Scherbe habe,
Si te reflejo desde otro trozo,
este se vuelve otra entrada.
Si te reflejo desde otro trozo,
este se vuelve otra entrada.
Si tu te reflètes autre part,
ça en fait une de plus.
Se riesco a farti riflettere in un altro frammento,
Se eu conseguir te refletir
com um outro caco do espelho,
Я могу затянуть тебя через 
любой осколок, в котором только увижу!