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'm going to go get whatever 
that thing is first!
!أنا ذاهبٌ لإحضار ذلك الشيء أيّاً كان أوّلاً
Ich hole mir erst mal,
was auch immer das ist.
Sea lo que sea esa cosa,
iré por ella primero.
Sea lo que sea esa cosa,
iré a buscarla primero.
Je vais aller récupérer
ce que vous cherchez !
Nel frattempo andrò a prendemi
ciò per cui siete venuti, qualunque cosa sia!
Eu vou me preocupar em
conseguir essa tal coisa primeiro!
Так что сперва 
я стяну то, что вы ищете!