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 he's stepped outside, 
there's a chance I might survive!
!الآن بعد أن خرج، هنالك احتمال أن أنجو
Und da er nun fort ist, habe ich
die Chance, noch zu überleben.
Ahora que salió,
tengo más probabilidades de sobrevivir.
Ahora que salió,
tengo más probabilidades de sobrevivir.
Et en étant de l’autre côté,
il me donne une chance de survivre !
Ora che è uscito fuori,
ho una possibilità di sopravvivere!
Agora que ele saiu,
eu tenho uma chance de sobreviver!
Зато теперь у меня есть
все шансы остаться в живых!