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...
Extract the serum from this 
snake and inject it into me!
استخرِج المصل من هذه الأفعى
!واحقنه في داخلي
Entziehe der Schlange
das Serum und injiziere es mir!
¡Extrae el suero de la serpiente
e inyéctamelo!
¡Extrae el suero de la serpiente
e inyéctamelo!
Récupère le sérum du serpent
et injecte-le-moi !
Estrai il siero da questo serpente
e iniettalo dentro di me!
Extraia o soro dessa cobra
e o injete em mim!
Извлеки из её крови иммунную сыворотку