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 stopped it! 
Now I just need to take this arm...
أوقفتُها! والآن كلّ ما عليّ هو أخذ
...هذه الذّراع
Ich habe sie aufgehalten.
Dann muss ich nur noch diesen Arm
¡Lo detuve! Solo debo llevar este brazo…
¡Lo detuve! Solo debo llevar este brazo…
L'ho fermato! Ora devo solo
trascinare questo braccio...
Eu o detive...
Agora é só tirar o braço dele...
Не достал! А теперь я отправлю руку…