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...
If he gets outside, the virus 
will be separated from him!
!إذا خرجَ فإنّ الفيروس سينعزِل عنه
Wenn er hinausgelangt,
wird er vom Virus getrennt!
¡Si sale, el virus se separará de él!
¡Si sale, el virus se separará de él!
Se uscirà dallo specchio,
riuscirà a separarsi dal virus!
Se ele sair, ele estará livre do vírus!