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...
The most important thing 
is to fulfill the mission.
أهمّ شيء هو إنجاز المهمّة
Das Wichtigste ist,
dass die Mission erfüllt wird.
Lo más importante
es cumplir con nuestro deber.
Lo más importante
es cumplir con nuestro deber.
Le plus important,
c’est d’accomplir notre mission !
La cosa più importante
è portare a termine la missione.
O importante é conseguir
cumprir a missão.
то мы выполним приказ, 
а это самое главное!