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 we just stab this in with the strength 
of a four-year-old child, it's pretty easy.
،إذا حشرنا هذا بقوّةِ طفل يبلغ 4 أعوام
يُمسي الأمر سهلاً للغاية
Es bräuchte nur die Kraft eines Vierjährigen,
um das dir so reinzuschieben.
Será sencillo enterrarte esto incluso
con la fuerza de un niño de cuatro años.
Será sencillo enterrarte esto incluso
con la fuerza de un niño de cuatro años.
Même un gosse de 4 ans
Perfino un bambino di quattro anni 
riuscirebbe a spingerti questa cosa in gola.
Até uma criança de 4 anos teria
força o bastante pra te matar assim.
Вогнать осколок в горло 
сможет и четырёхлетний пацан.