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 just need the last one 
standing to tell me.
أحتاج لآخِر من يبقى حيّاً أن يخبرني
Ich muss nur vom letzten
eine Antwort bekommen.
Basta con que haga hablar
al último de ellos.
Basta con que haga hablar
al último de ellos.
Il suffira que le dernier debout
crache le morceau.
Mi basterà che me lo dica
quello che rimarrà per ultimo.
Se o último sobrevivente me contar,
pra mim já serve.
Главное, чтобы хоть 
один из вас дал мне ответ.