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...
First is disposing of Giorno 
after I get the key.
الأوّل هو التخلُّص من جورنو بعد أخذ المفتاح
Zum einen könnte ich erst den Schlüssel
an mich nehmen, bevor ich Giorno beseitige.
La primera es conseguir la llave
y luego deshacerme de Giorno.
La primera es conseguir la llave
y luego deshacerme de Giorno.
Le premier, récupérer la clé
puis liquider Giorno.
Posso liberarmi di Giorno
una volta presa la chiave.
Posso pegar a chave primeiro
e me livrar do Giorno depois...
Первый: сперва завладеть ключом, 
а потом избавиться от Джорно.