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...
That's for the best, though. 
He's going down, no matter what.
لكن هذا لصالحِنا. سوف يسقط مهما حدث
Aber das ist gut so. Er wird
auf jeden Fall sein Ende finden.
Pero así es mejor.
Caerá a como dé lugar.
Pero así es mejor.
Caerá sea como sea.
C’est mieux comme ça.
De toute façon, il est condamné.
Però è stata la cosa migliore.
Alla fine comunque ne uscirà sconfitto.
Mas foi melhor assim.
Ele vai morrer, de um jeito ou de outro.
Но это ничего не меняет.
По обе стороны его ждёт одна судьба.