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 doesn't just mean 
we failed the mission.
هذا لا يعني فقط أنّنا فشِلنا في المهمّة
Damit wäre nicht nur
unsere Mission gescheitert,
Y no solo marcará
el fracaso de la misión,
Y no solo marcará
el fracaso de la misión,
S’il y arrive,
non seulement on aura échoué,
E questo significherebbe
non solo fallire la missione...
Isso significa que não só
a nossa missão será um fracasso,
Мало того, что я 
не справлюсь с приказом,