Flandre stays in the basement of her own volition because she respects her sister and looks up to her as a motherly figure. It would be easy for her to break out at any time. Remilia holds frequent parties, and Flandre comes out at these times as well; she isn't as bound as most people think she is. Remilia is slowly training her to use her powers on a smaller scale as to not become a menace to the peaceful world they moved to. She doesn't tell Flandre that she eats people because Flan would likely not know any better and flatten the human villages without a second thought. Without human blood to feed on, they would likely both die; so this is necessary for their survival.
She taught her the spell card system, which she actually uses when playing with the heroines, and Remilia fated a meteor to enter Gensokyo's orbit so that Flan could destroy it. While many people think of Remilia as a spoiled bitch, she may be, but she is looking out for her sister and is keeping her powers under control in the best possible way she can. Whether is was out of fear for her life and for others is a different matter entirely. Power corrupts, and absolute power corrupts absolutely. If Flan wasn't insane to begin with, her powers could have likely driven her to her current state, as well as keeping out of contact with people for many, many years. Social contact is the main influence for people to mature, which Flan did not have. As such, she is burdened with a childish mind, and when she's in the basement her imagination can run wildly, which would also be a large factor.
Flan's behaviour is much like a child; and what would a child do with the power to destroy? They would end up weaving a path of destruction, most likely. Calling it ADHD would be a misdiagnosis. Her psychological and social development is stunted and her powers make her dangerous, but she is not insane. The respect she holds for her sister at the very least shows that she isn't just a maniac. Without her destructive powers, what would she be? Just an "everyday" vampire. There would be no need to keep her in the basement, she would not be socially underdeveloped, and she would probably sit at Remilia's right hand. She would not be dubbed insane.
Thus I conclude that her powers are the entire problem, Flan as a result has a childlike psyche and she is in fact, not insane.
Flandre stays in the basement of her own volition because she respects her sister and looks up to her as a motherly figure. It would be easy for her to break out at any time. Remilia holds frequent parties, and Flandre comes out at these times as well; she isn't as bound as most people think she is. Remilia is slowly training her to use her powers on a smaller scale as to not become a menace to the peaceful world they moved to. She doesn't tell Flandre that she eats people because Flan would likely not know any better and flatten the human villages without a second thought. Without human blood to feed on, they would likely both die; so this is necessary for their survival.
She taught her the spell card system, which she actually uses when playing with the heroines, and Remilia fated a meteor to enter Gensokyo's orbit so that Flan could destroy it. While many people think of Remilia as a spoiled bitch, she may be, but she is looking out for her sister and is keeping her powers under control in the best possible way she can. Whether is was out of fear for her life and for others is a different matter entirely. Power corrupts, and absolute power corrupts absolutely. If Flan wasn't insane to begin with, her powers could have likely driven her to her current state, as well as keeping out of contact with people for many, many years. Social contact is the main influence for people to mature, which Flan did not have. As such, she is burdened with a childish mind, and when she's in the basement her imagination can run wildly, which would also be a large factor.
Flan, if let outdoors, would be able to catch her own food easily, as she would just kill random humans as she pleased, whether she's insane or not. It's her social ineptitude that makes her unable to care for herself and have such a dependence on her sister; as it was probably planned by Remilia in the first place. If Flan's power was that unstable, then making her dependent on the only person who can control her is the perfect way to keep her in check. Flandre respects Remilia more than anyone else and Remilia takes advantage of that to keep her still.
Flandre is obviously more childish than Remilia. If not shown by the dialogue in-game that Remilia is actually fairly civilized as opposed to Flandre, it's also shown in PMiSS and BAiJR. Yes, Remilia is childish, but only in the sense that she might whine or cry when something bad happens, or that she's a bit self-centered. Mostly, she's fairly mature and put-together. The entire reason people call Flan insane to begin with is that nearly any time she's introduced, she will destroy something because of her childlike state of mind and unstable powers, and nobody can see beyond that because they're afraid of her. The difference between Remilia being insane and Flandre being insane isn't because Flandre actually is insane, but because Remilia has been exposed to social interaction her entire life (500 years is enough to act fairly civilized, one would think) and her powers aren't destructive enough to blow up a meteor.
The reason Flan doesn't mind being down in the basement all the time is because ever since she was five she's been locked up. It's what she has done her entire life. She is let out on some occasions, but these are probably regarded as privileges and a special treat, not the basement being a punishment. She looks up to her sister and does what Remilia thinks is best for her, which is probably true.
Flan's behaviour is much like a child; and what would a child do with the power to destroy? They would end up weaving a path of destruction, most likely. Calling it ADHD would be a misdiagnosis. Her psychological and social development is stunted and her powers make her dangerous, but she is not insane. The respect she holds for her sister at the very least shows that she isn't just a maniac. Without her destructive powers, what would she be? Just an "everyday" vampire. There would be no need to keep her in the basement, she would not be socially underdeveloped, and she would probably sit at Remilia's right hand. She would not be dubbed insane.
I conclude that her powers are the entire problem, Flan as a result has a childlike psyche and she is in fact, not insane.
Here's another way of thinking about it. Remilia can manipulate fate. Flandre obeying her and staying in the basement is accomplished because Remilia uses her powers to control Flan' fate. Flan never questioning what she eats, Flan coming up only when Remilia wants her to and respecting Remilia in general: is all controlled by her powers. Why do you think they moved to Gensokyo? Why does Flan only escape from the mansion immediately after the heroines defeats Remilia, and the heroines just so happen to be there on the day Flan gets loose? Think about it.
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