I really love Gurren Lagann and I wanted to write something from Messenger!Nia's point of view. It seems like it would be horribly horribly lonely to be her, even more so if you had a memory of what something felt like but couldn't feel anything for yourself. I hope I managed to convey that!
Irregular
In one instant she was born, and in that same instant she gained all the knowledge and memories of her host body, so many sensations flooding her mind at once and rendering her paralyzed.
In the next instant she gained all the knowledge that had been planted inside her when she was just a single strand of DNA. Just a moment ago she had understood joy and pain and love- now she only knew it, the way one knew that the sky was blue or the Earth was round. They were irrelevant facts of the Spiral Races, of which she was no longer, and they would make no difference to the destruction of the Spirals. Now she only saw the foolish ambitions of these humans, and their imminent demise. The heartbroken expression of the young man's expression as she faded into the sky meant nothing to her.
She saw things now in terms of percentages, probability, and despair. Human beings were fools driven only by instinct, ignorant and uncaring of the fact that they would destroy everything, and because of that they must be eliminated. The job of the Anti-Spirals was to prevent the Spiral Nemesis from existing. The job of the Messenger was to inflict absolute despair. She did only her job, and thought of nothing else.
For a while, she didn't realize it. Maybe it was just the shock of the body's conversion and the birth of a new mind that the host lay dormant. But the original consciousness still existed inside her, and when it woke again it wracked her body with these things called "emotions" that she knew existed but could not comprehend. It was not data that she could analyze.
She went to the man again, hoping perhaps to hurt the other mind. But instead it only swelled with the things called love and fear and a mix of so many others, and if she could have felt something she would have been disturbed. This being should no longer exist, she thought. How is it that it is able to hold such ties with this warrior? If that is possible, then…
As time passed, she continued to think about it. Her body was still human, and the original mind still had some effect on it. She would find something in her chest twisting and pounding sometimes, and every once in a while she would catch just the faintest glimpse of something she had known for just an instant.
And then, as she spread her limbs to block Cathedral Terra's control system with a massive drill tip at her throat, she did more than know- she understood. She tried to deny it with all her being, but she understood. Perhaps it was only because of that all-too-brief moment before her Anti-Spiral memories awoke. She understood now the compulsions to visit the man, those strange sensations inside her chest and throat.
She had someone else's memories, someone else's life, planted in her brain, along with their joy and pain and love. They didn't belong to her, and when the man looked at her with that pleading expression it wasn't her he wanted to see.
She still had her mind, and she knew that the appropriate response was loneliness, but she couldn't muster up the feeling of it. And the other mind, the original mind, was writhing and struggling and doing all it could to take over.
She didn't quite know why, but she felt then that maybe she was the being who should no longer exist. Irregular, she thought to herself.
And then she let go.