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...
Now that it's in shards, I have even 
more entrances to grab you from!
الآن بعد أن تحطّمَتْ إلى قِطع، باتَ لديّ
مداخل أكثر حتّى لأمسِك بك منها
Und nun in Scherben habe ich mehr Orte,
von denen aus ich dich zu mir zerren kann.
¡Más trozos significan más entradas
por las que puedo arrastrarte!
¡Más trozos significan más entradas
por las que puedo arrastrarte!
Un miroir morcelé, ça me fait
encore plus d’ouvertures !
E ora che è in frantumi,
ho molti più punti da cui catturarti!
E agora que está estilhaçado,
eu tenho muitas mais entradas!
А следовательно, чем больше осколков, 
тем больше путей на другую сторону!