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...
To make sure you can't move about, 
I'll only allow half of your body to come in.
لأحرص على ألّا تتمكّن من الحركة، سأسمح
لنصفِ جسدك فقط بالدخول
Und damit du keine unnötigen Mucken machst,
erlaube ich nur deinen halben Körper hier.
Para evitar que te muevas,
solo traeré la mitad de tu cuerpo.
Para evitar que te muevas,
solo traeré la mitad de tu cuerpo.
Tu vas rester sagement en place.
E per assicurarmi che tu non possa muoverti
liberamente, farò entrare solo metà del tuo corpo qui.
E pra garantir que você vai ficar aí parado,
vou permitir que apenas metade do seu corpo entre.
И чтобы ты точно никуда не сбежал, 
я позволю войти только половине тела!