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The mirror world extends to a few hundred meters of ground, with the original mirror entrance at the center. So there is a limit to how far one can travel in the mirror world based on their original point of entry. At this border, I personally imagine a black void blocked by an impassable barrier. Not even Illuso can get past this “barrier” as it is the limits of his stand’s ability.
You can think of the mirror worlds created here more like pocket dimensions created by Man In The Mirror itself, not a preexisting realm of existence only accessible by Man In The Mirror. If the latter were true, Giorno, Abbachio, and Fugo would have still been there after Illuso’s death. Because when Illuso and his stand were killed, all of its effects were undone. This including the creation of the mirror world. So while Polnareff can tease Kakyoin about the whole “no such thing as a mirror world,” Kakyoin is still technically correct in the fact that there really isn’t one. It was just created at the time of Man In The Mirror existing. high fives kakyoin and hits the griddy while polnareff sobs on the floor
The nature of the mirror world, being a “Realm of Death” where no living thing exists unless Illuso allows it permission to enter by dragging it in, applies to every single living microorganism or anything with life energy like a stand. This includes viruses and bacteria. So although the mirror world is rightfully eerie and uncanny being devoid of life, it is a sterile environment that the team uses for medical prep and short-term recovery when necessary. However, living organisms existing within or on someone that Illuso drags in will be carried in. Illuso has to be aware of it to prevent it from entering, and removing or preventing it from entering is usually impossible because of the energy it would take to strip a whole body of every single microbe already on it. When leaving the mirror world infected with Purple Haze’s virus, he had to cut off his whole arm rather than waste energy and time to try and remove the virus from his body in a method other than amputation.
This “amputation method” can be used on victims, though it takes a bit more energy than simply dragging them or only parts of them through the mirror world.
Physical items and structures can only be moved by Man In The Mirror or Illuso. Physical items inside of the mirror realm cannot be taken out of it. Attempting to do so to make a duplicate of a physical item will leave Illuso only empty-handed. So theft and attempts to get two of a single item are impossible this way. If Illuso wants to steal something, he’ll have to retrieve it physically in the real world and carry it through the mirror world to a second location.
Changes or alterations made to the physical area in the mirror world will not reflect in the real world. Blood stains, objects being moved or destroyed, or even a dead body brought into the mirror world will not manifest in the real world. The same is not true for how physical damage is done in the real world, as then it will be reflected in the mirror world.
The laws of gravity and relativity still apply to the mirror world besides this.
Illuso can bring objects into the mirror world, including ones attached to a person’s physical form. The whole clothing attached to a body is where I differentiate from canon, as I find it a liiittle silly that clothes worn on the body in the mirror world are made of spiritual energy… like are you meaning to tell me everyone woulda been naked except Illuso doesn’t wanna get flashed so he takes care to give em clothes made outta spiritual energy? I just believe that Illuso gives the clothes and objects on the body full permission to enter as well by default, or that it’s something he doesn’t need to pay too much attention to detail on. Like if he saw a knife on someone’s belt, he could definitely be like “drag them in but leave the knife out” so THEN they’re defenseless. Illuso CAN be tricked, after all. His stand doesn’t give him a full stat list or overview of whoever the victim is, which is how he was tricked by Abbachio and Moody Blues and Giorno with the virus.
Dragging people into the mirror world takes energy. Dragging one person in is easy, but dragging several people in rapidly takes up too much energy. This usually disqualifies him from hits that require ambushes on large groups of people. How this exactly takes a toll on him, I like to imagine he gets really light headed and physically weak, and just becomes incapable of bringing anyone or anything between the mirror realms for a while.
However, only the action of bringing people in and out of the mirror world takes energy. Leaving or having people in the mirror world does not affect Illuso. Illuso can leave abducted targets to die of starvation/thirst, though he prefers to kill them himself. People can be kept inside of the mirror world for however long they can, so long as they have supplies to live off of. Basically a prison that only Illuso holds the keys to.
Illuso can freely go between the mirror world and the real world whenever he wants at no expense to his energy. Risotto always cautions him to reserve his ability only for when it is most needed. Risotto believes that a power like Illuso’s could lead him to getting too big in the head, so he does his best as a leader to stop Illuso from being too proud. Obviously doesn’t work out that much, though.