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...
If you had, my Stand's hand 
would've stopped moving.
لو فعلت، لتوقّفَتْ يد الستاند الخاص
بي عن التحرُّك
Denn dann hätte sich die Hand
meines Stands nicht weiter bewegt.
Así hubieras detenido
la mano de mi Stand.
Así hubieras detenido
la mano de mi Stand.
Au moins, la main de mon Stand
ne bougerait plus !
Se l'avessi fatto, la mano del mio Stand
avrebbe smesso di muoversi.
Se tivesse feito isso,
teria detido a mão do meu Stand.
и тогда рука стенда 
уже никуда бы не убежала!