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...
So he's inside the mirror...
...إنّه داخل المرآة إذاً
Er ist also in dem Spiegel.
Está en el espejo…
Está en el espejo…
A quanto pare si trova
all'interno dello specchio...
Então ele está dentro do espelho...