In Stardust Crusaders, when Polnareff and Kakyoin are being pursued by J. Geil’s stand, Hanged Man, the former expresses his annoyance that Hanged Man keeps attacking from reflections even though they tried breaking a mirror into pieces. Polnareff doesn’t know how they’re expected to attack Hanged Man if they can’t go inside the “mirror world,” to which Kakyoin informs Polnareff that there is no such thing as a mirror world…until Part 5 comes around. Golden Wind features a minor antagonist named Illuso who has a stand called Man in the Mirror, which allows him to access a mirror world. Illuso is also able to drag anything into the mirror world by using any reflective surface, no matter how small. The mirror world is just like the real world except reflected, e.g., letters in the mirror world are reversed. To be fair, this could be chalked down to Kakyoin simply not being knowledgable about the existence of this strange world.
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