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...
it doesn't have much destructive power. 
That is indeed one of its traits!
!هذه بالتأكيد إحدى صِفاته
So ist er nun mal drauf.
Es una de sus características.
Es una de sus características.
et tape donc moins fort
qu’un Stand ordinaire.
non ha una grande potenza fisica.
In effetti è una delle sue caratteristiche!
e não tem muito poder destrutivo.
так что сейчас он практически безвреден.