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...
You sure have a loyal companion.
لديك شريكٌ وفيّ حقّاً
Du hast schon treue Kumpanen.
Tu compañero sí que es leal.
Tu compañero sí que es leal.
T’as un sacré camarade,
fidèle parmi les fidèles.
Hai davvero un compagno leale.
Você tem um colega bastante leal, mesmo!
Хорошего ты выбрал себе товарища!