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...
Looks like your Stand 
itself isn't that strong.
يبدو أنّ الستاند الخاص بك ليس بتلك القوّة
Dein Stand für sich scheint
nicht sonderlich stark zu sein.
Parece que tu Stand
no es muy fuerte de por sí.
Parece que tu Stand
no es muy fuerte de por sí.
Son Stand met pas
de torgnoles trop violentes.
A quanto pare il tuo Stand
non è granché forte, fisicamente.
Parece que o seu Stand
não é particularmente poderoso.
Похоже, твой стенд не представляет
 особой угрозы в бою.