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 thought he was some newbie 
that I just couldn't figure out,
...حسبتُه مجرّد عضو جديد لا أعرف عنه شيئاً
Ich dachte, er wäre so ein Frischling,
der von Tuten und Blasen keine Ahnung hat …
Creía que era un novato
sin idea de nada…
Creía que era un novato
que no sabía nada…
Je le prenais pour un débutant
qui enchaînait les gaffes,
Pensavo che fosse solo un novellino
un po' difficile da inquadrare...
Pensei que era apenas um novato meio tímido...
Я думал, он ничего не знающий сопляк, 
которого мне не понять.