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...
is when I'm following the orders 
of something great and absolute.
يأمرني شخصٌ مُطلَق وأكثر عظَمة بكثير
wenn eine größere Instanz
als ich es bin mir Befehle erteilt.
es cuando recibo órdenes
de algo mucho más grande que yo
es cuando recibo órdenes
de algo mucho más grande que yo
c’est d’obéir aux ordres
de grands hommes absolus.
sono quelli in cui seguo gli ordini
di un'entità superiore, assoluta.
quando alguém maior e mais
importante me dá uma ordem.
когда мне даёт приказ некто
могущественный, кого я не могу ослушаться.