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...
Shattering the mirror actually 
worked in my favor, Abbacchio.
كان تكسير المرآة لصالحي في الواقِع
يا أباكّيو
Den Spiegel zu zerschlagen,
hat mir in die Karten gespielt, Abbacchio.
Me hiciste un favor
al romper el espejo, Abbacchio.
Me hiciste un favor
al romper el espejo, Abbacchio.
Réduire le miroir en pièces
produira l’effet inverse.
In realtà mi hai fatto un favore
rompendo lo specchio, Abbacchio.
Quebrar o espelho acabou
me favorecendo, Abbacchio.
Разбив зеркало на мелкие куски, 
ты сделал только хуже, Аббаккьо.