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...
I win! Thanks for telling 
me where the key i—
—ربِحت! شكراً على إخباري عن مكان المفتاح
Damit habe ich gewonnen. Danke schön,
dass du mir gezeigt hast, wo der Schlüssel i…
¡La victoria es mía!
¡Gracias por decirme dónde está la…!
¡La victoria es mía!
¡Gracias por decirme dónde está la…!
J’ai gagné !
Merci de m’avoir montré la clé !
Ormai ho vinto! Grazie per avermi
mostrato dov'era la chia—
Eu venci! Obrigado por me
mostrar onde estava a chave!
Я победил! Благодарю, 
что указал мне на ключ!..