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...
There's no way you could grab Mirror Man's neck
when I'm the one who dragged you in here!
من المُحال أن تتمكَّن من إمساكِ عنق ميرور
!مان بعد أن سحبتُك إلى هنا
Du könntest nie Mirror Mans Hals greifen,
nachdem ich dich herübergezogen habe!
¡No deberías poder sujetar
el cuello de Mirror Man una vez aquí!
¡No deberías poder sujetar
el cuello de Mirror Man aquí!
Un mec aspiré devrait pas
pouvoir y parvenir !
Sono stato io a trascinarti qui dentro, non puoi 
essere riuscito ad afferrare il collo di Mirror Man!
Dentro do meu mundo, você não tem
como agarrar o pescoço do Mirror Man!
Я затянул тебя самого! Человек не может
 прикоснуться к моему стенду!