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 just made a prediction 
and followed through.
...توقّعتُ شيئاً وتابعتُ بناءً عليه فحسب
Ich habe nur eine Vermutung angestellt
und mich daran gehalten.
Solo hice una predicción y actué acorde…
Solo hice una predicción y actué acorde…
Ho solo fatto una previsione
che si è rivelata azzeccata...
Eu só previ uma coisa e a vi desenrolar...