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 starting to understand 
the enemy's ability.
...بدأتُ أفهم قدرةَ العدوّ
Ich verstehe langsam
die Fähigkeit des Feindes.
Comienzo a comprender su poder.
Comienzo a comprender su poder.
Ça y est, j’ai saisi.
Sto cominciando a capire
i poteri del nemico...
Estou começando a entender
o poder inimigo...
Теперь я знаю, как устроена его сила.