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 told you! Anything that 
reflects becomes an entrance!
أخبرتُك! أي شيء يعكس يُصبِح مدخلاً
Wie ich es sagte: Jede spiegelnde Oberfläche
wird zu einem Zugangspunkt.
¿No te lo dije? Cualquier trozo
que refleje se vuelve una entrada.
¿No te lo dije? Cualquier trozo
que refleje se vuelve una entrada.
Je t’avais prévenu,
Te l'avevo detto! Tutto ciò che
lo specchio riflette fa da punto d'ingresso!
É como eu falei: tudo que meu
espelho reflete vira uma entrada!
Ты забыл? Всё, на чём есть отражение, 
служит входом в мой мир!