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...
And I'll give you, and only you, 
permission to come into the mirror!
وسأمنحُك، وحدك فقط، الإذن للدخول
!إلى المرآة
Und dir, und zwar nur dir, erlaube ich,
zu mir in meinen Spiegel zu kommen.
¡Y ahora permitiré que tú,
y solo tú, entres al espejo!
¡Y ahora permitiré que tú,
y solo tú, entres al espejo!
Viens, je t’y autorise !
Darò a te, e solo a te,
il permesso di entrare nello specchio!
E eu vou dar permissão de entrada
exclusivamente pra você!
Я разрешаю тебе…