Thought: With a heart not working entirely correctly, Koishi is effectively some sort of sufferer of Magical Asperger's Syndrome.

... I, uh, I'll just let that sink for a moment.

The point is, she lost the ability to just feel her way through human interaction, all those instinctual tidbits that just come naturally to most people. She doesn't interact much with people, but she walks the earth observing them, and can even make herself invisible while doing so. It's not hard to imagine that her apparent interest in psychology stems from gaining a new perspective on people's behavioral patterns... and/or from just observing a bunch of people and trying to systemize her observations as a hobby.

... Sorry, my thoughts are probably pretty incoherent right now. I need to catch some sleep.

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Warning: The follow write-up is written without much pre-planning or coherence because the writer deemed it only fitting for this character. Caution is advised.

The intelligent beast. The loveless seeker of love. They call her... Nobody.

A dreamer who believes in true love! They keep talking of it in songs and stories! Oh, how nice it must be to feel this, this love! Sister, where is this love? Why is all we ever get, hate? Hate. Hate hate hate HATE

The elder sister Satori was more resilient than her younger sibling. She was content with her house and her pets and her lively Koishi keeping her company. Koishi, however, suffered from a severe case of optimism. Idealism, if you will. Love-sickness for love. Thirst for all the adventures this outside world has to offer. She cannot quench this thirst in this cage, this cage within a larger cage! She can burst free! She can burst free of the cage of her existence as an outcast, and then she'll burst free from this house, from this land, and then the whole world will be hers to take, why, she will grow to the size of the sky just to embrace all of the world in her arms, so says she with her heart aflame and in tears.

So said she, and took the knife.

Things don't work that way, Koishi.

Koishi.

Why did you kill yourself?

Your body is still alive. Your brain is still working. But this Koishi isn't the one I grew up with. Oh, sister, if your soul is still around, if I can somehow make you return to the way you were. I want to give you all the love and all the happiness this world has to offer, so please. Please. Please come back to me...

The eye was shut, and thus was the heart. What remained of Koishi was superficially everything but the opened third eye, and yet, Koishi herself is gone. Without the heart, the feelings, the part of the soul that binds her to this world, what is left is a raging intellect with memories of what it means to be a person.

It would be wrong to say that she is inhuman, or inyoukai, as it were. But her regard for the people around her. Her self-control. Her desires and dreams. All of these dissipated into all cardinal directions. She wanders around aimlessly, acts erratic, sleeps in the mud, won't hold a coherent conversation, eats things she finds on the ground, lets herself drop without warning, goes for days without food. Even an animal has instincts and a social nature, but Koishi, the intelligent beast, has no such things. There exists the world, and there exists her. Her two eyes see the world, but the world is something apart from her. A movie, a simulation, shadowplay. There is so much to do. What does this do. What do these people say. What do these kinds of creatures act like. She wanders. She watches.

It's not that she is dangerously insane. She is harmless. A hedonist, a slave to her freedom, but at the same time, practically unaware that the world around her is something that can be changed by her. Like the player character in a game world, she wanders to discover everything, losing interest as soon as gaining it, not expecting the "game" to give her anything that it doesn't give her on its own. She is harmless. But she is no longer the Koishi her sister loved. Satori can no longer read her mind. She cannot talk to her. She often won't see her for weeks upon weeks.

The young Koishi, bursting with life and love, thinks herself lucky, as though she has gained what she dreamt for – the love of everyone, the ability to venture wherever she wants to, the ability to embrace the whole world. Yet what she gained is indifference, as people can barely sense her presence anymore; the prison of freedom, as she can go wherever, but has no reason to be in any one place; and the only thing filling her life, observing the world through a looking glass, making connections, taking notes, creating formulas for the AI of this artificial world in front of her, in her never-resting magnificent and lonely brain.

What will happen after she meets that human, that person who broke the patterns she created, that creature who seems like more than another NPC... that thing that might be another... one like her?

Return from that odyssee, Koishi. Absolute freedom is not for the living.

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Imagine that everyone fears you, and you know that to be completely true. Imagine knowing every terrified, contemptuous, hate-filled, bitter thought that is directed towards you, that invariably appears whenever you do. You will never be loved while you can see beneath their skin to see the crawling filth within their hearts. Everyone tries to plaster over the ugliness and sin, hide it away, and they hate you because you can see it. And you can't take one more moment of it. Your sister loves her pets, and no one will ever ever love you.

One night, you made it so that no one would ever have to fear you again. Never ever never. And your sister walks in and sees the blood trickling down your forehead and you smile because everything's FIXED now and now you can't SEE all the hate and lies and fear.

But you can taste them now. You can't read the book blind but you can feel the pages and understand what's behind the words. And nothing changes nothing really changes because you still feel everything you were running from. So you keep running and running and running so long and so far, here and there and yonder and hither and home.

People can't see you, but you can't see them. So you grope along, trying to find what you lost, because then- then-

You don't know. You're scared of seeing everything again. The blind can't see if they won't open their eyes, and you WON'T NEVER AGAIN because if you can't feel you can't get hurt again and cry yourself to sleep at night when sister isn't listening because everyone hates you-

Imagine that you're blind, wandering through a half-known world, free of everything except what's in your own head. And you can't feel...

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So if you look into Koishi's eyes, those of a person who has severed that connection, what would you see? An empty void? Something that looks like a doll's eyes, staring but lifeless? Or some sort of madness beyond even a lunar rabbit's lunatic eyes?
It's more like you don't look into her eyes, you look at them, and you don't see anything "in" them, like friendliness or uncertainty or whatever else you might observe from the eyes of a normal person... and that just kinda throws you off and you don't know, why.

ZUN does a magnificent job of fitting his themes to his characters, so I try to get my sense for Koishi from "Hartmann's Youkai Girl." First off, it's broken - the tempo and tone lurches. It repeats, but is arrhythmic, lurching. It's fast and frenzied in parts, but it never goes anywhere, and slips into a sort of background noise, frantic but calming. There are moments when it seems to be building towards something, anticipation grows towards a climax, but it falls back into old patterns. But the pressure mounts, intensity builds, and then there is a moment of poignant clarity, a sustained melody that breaks free from the roiling chaotic accompaniment, but only for a moment. Then the cycle continues.