Different
Intellectually and emotionally, Reiko was not like other parasites. It was almost like she was more developed than others in her species. While other parasites largely acted on instinct, throughout the series Reiko had proven that she was able to calmly think things through and make decisions that were not only best for her, but for her entire species as well. Keeping Shinichi alive is a big example of this. Sure, he was killing parasites left and right, but Reiko saw that Shinichi must have had an advantage that made him a danger. She knew that something was making Shinichi stronger. She wanted to study and learn more about him so that the future of her species would be better. That's why she was constantly studying, observing, and experimenting with everything.
I'm working for the future potential of all of us.
She firmly believed that the parasites had a purpose other than ridding the Earth of humans. She was alone in thinking that. Other parasites felt like they were the Earth's antidote to environmental destruction caused by humans. I think it saddened her.
Sigh. So I'm raising poison inside my body?
How frail and incomplete we are.
Nevertheless, Reiko pushed forward with her studies. She kept asking questions. She wanted to know where the parasites came from, why they were there, and what kind of future there was for them. It bothered her that so little was known about her species. Even the baby she produced was fully human. What was their function if they couldn't even reproduce?
I don't know if Reiko expected to find any answers to her questions in humanity, but unlike the other parasites, Reiko was fascinated with humans. She knew that in order for the parasites to survive, they'd have to find a way to coexist with humans. That's probably why she was so keen on humanity. In beginning to understand humans, Reiko started to think more human, and that made Reiko different. Kusano, another parasite even picked up on her different way of thinking.
Are you really one of us? You see things a little differently...
Kusano blamed Reiko's baby for her changing, and he was probably right, at least partly. When her baby's life was put in danger, Reiko realized that she actually cared about her baby. That was something that was inconceivable to a parasite.
One of the most surprising differences in Reiko was revealed right before she died. She told Shinichi that she had only killed 38 humans, and that most of them were for food. She said that's really low for a parasite, and that she tried to make up the difference with normal human food to prove that parasites didn't need to eat humans to survive. I wondered why that was important to her, and why she even cared. It could be because her son is human, or because in learning about humanity, she began to sympathize with humans. I think it's because she wanted a purpose, though. She wanted to be more than a man-eating monster. Even though Reiko couldn't change her species' behavior, she became more.