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 sure you're very familiar with this arm!
!أنا متأكّدٌ بأنّك تألف هذه الذِّراع جيّداً
Ist doch ein Arm, der dir
bekannt vorkommen dürfte.
De seguro te resulta familiar.
Seguro que te suena.
Tu le connais bien, hein ?
Mi sa che è un braccio
che conosci molto bene!
Aposto que você conhece
bem este braço!
Уверен, она тебе хорошо знакома. 
Узнаёшь?