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...
Shards of the mirror that 
Purple Smoke shattered...
...قِطع المرآة التي حطّمَها باربل سموك
Scherben des Spiegels,
den Purple Smoke zerschlagen hat …
Los trozos del espejo
que rompió Purple Smoke…
Los trozos del espejo
que rompió Purple Smoke…
Les fragments du miroir
sont alignés…
I frammenti dello specchio
rotto da Purple Smoke...
Cacos de vidro do espelho
que o Purple Smoke estilhaçou...
Кто-то собрал осколки 
разбитого зеркала.