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...
You should've sliced open my 
throat when you had the chance.
كان ينبغي أن تشقّ حنجرتي عندما سنحَتْ
لك الفُرصة
Hättest mir die Gurgel durchschneiden sollen,
als du die Chance dazu hattest.
Debiste degollarme
cuando tuviste la oportunidad.
Debiste degollarme
cuando tuviste la oportunidad.
T’aurais peut-être bien fait
de m’ouvrir la gorge.
Avresti dovuto tagliarmi la gola
quando ne hai avuto la possibilità.
Você devia ter cortado minha garganta
quando teve a chance.
А ведь ты мог просто 
перерезать мне горло,