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i dunno maybe she just wants books despite them being expensive is because its her source of comfort and she grew up with it in her past life, and maybe the chores are hard for her because she's sickly and frail, on top of having the consuming and not even being 7 years old yet. Despite causing trouble (half of which are due to her illness anyway) shes making more money than her parents and gives them money on top of paying for a temporary cure for her illness. i think she's at least allowed to complain if she has to go through all that.
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i think her mental age has been affected by her physical age. it is better to think of her as a mature child. I understand what you mean. From the perspective of an adult it is strange that she is being inconsiderate to the financial and physical burden on her parents. however, as a child, she is not responsible for knowing how the world works, nor is she capable of taking care of herself. The way I see it, the same way a baby has the right to cry when they are hungry, she has the right to make her needs known as she is not capable of doing it herself. Also, as explained later, a lot of her behavior comes from her magically induced obsession with books. basically if she gives up on her goal of making books, she will die form magical consumption. As a defense for that she has been subconsciously single-minded about her goal. Making money is mostly incidental. as time goes on, she does try to take more responsibilities and help, but she is unable to.
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I mean when you live in present-day Tokyo where everything is basically everywhere, you have everything you ever needed in life, then you're suddenly thrown into a world that has none of these amenities you grew up with that you're used to having, I think one would be a little bratty in the beginning. She's trying to find resources to keep herself clean (since no one really showers as often in the world she's stuck in compared to the world she lived in, which is understandable since a lack of showering could lead to bad health), and tries to find the one thing she loves which are books. Won't deny that she is a pain to her parents in the beginning, however, at the time she just wasn't used to that world yet and she did have tantrums but when thinking about it, it would be frustrating to live in a world that you now have to reside in that has regressed technology you're not used to.
Because I can't see her as anything other than a selfish, bratty, child. You'd think being an adult, she'd realise that her family is poor, and thus unable to afford books, or any other luxuries for that matter. Not to mention, she always causes trouble, and doesn't listen to her family, throwing huge tantrums and refusing to do even the simplest chores. Jesus.