Shinichi
Shinichi should have been Reiko's enemy. Reiko was a parasite and Shinichi was a danger to her species. While she approached Shinichi with caution, I don't think she considered Shinichi an enemy, though. They had more in common than you would think.
Both Shinichi and Reiko were anomalies. While Reiko thought differently than other parasites and was more in touch with humanity, Shinichi was living with a parasite, Migi, who took over his right hand. Migi also made him stronger and faster. Shinichi's differences fascinated Reiko.
When she quit being Tamiya Ryoko, Reiko was going to kill Shinichi because she didn‘t see a reason to keep him alive, but she saw something in Shinichi other parasites weren‘t able to see.
You have a little mixed in. Interesting.
Ever since then, she had a fascination with Shinichi. Not only did she not harm Shinichi, she did what she could to keep other parasites from wanting to harm him too. She told them that he was powerful, and that there had to be a reason why. She convinced them that he was more useful to them alive, and she convinced them to let her watch Shinichi.
I think Reiko believed Shinichi was somehow important to the parasites' future. That's what she said when she asked Hideo to watch him but not harm him. She used several different methods to observe Shinichi. Hideo was just the first, a parasite who had taken over a boy Shinichi's age. She also hired a private detective, and teenagers to attack him. She even met Shinichi a couple times to exchange information, never intending to harm him.
When Shinichi and Reiko got together to discuss things, Reiko often brought up the questions she had with existence: why the parasites were there, what their purpose was, what kind of future they had, et cetera. I guess she expected that in one way or another, Shinichi would eventually be how she found the answers. To Reiko, he was the key in understanding their future, after all. While Shinichi didn't give her any answers, I think he was largely responsible for the changes in Reiko. In one of their first conversations, Shinichi asked her why she had to kill. He said that Migi lives off of Shinichi's own blood and regular human food. He proposes that she can too. I think that conversation left her with a lot to think about. She mentioned to Shinichi that flies know how to fly without being taught, and spiders know how to spin webs. Like they are just following orders, parasites do what their instinct tells them.
When I stole this human's brain, I received a single order. Kill and eat this species!
Throughout the series, Reiko tries to get other parasites to act against their instincts. I think she knew that they would never be able to coexist with humans if they didn't, and furthermore, they would have no future. Right before Reiko dies, she tells Shinichi how few humans she ate, and that she was trying to see if parasites could live without eating humans. That alone shows that Shinichi was at least partly responsible for Reiko's differences. He had somehow gotten to her.
It wasn't completely one sided, though, as Reiko changed Shinichi as well. Reiko handed Shinichi her baby before she died. Before Reiko's death, Shinichi was incapable of crying. Tears just wouldn't come no matter how sad he got. When Reiko died, however, and Shinichi was holding her baby, tears filled his eyes. Shinichi's own mother had recently died as well, and I think Reiko's death helped him finally mourn his own mother's death.
You're sad? Of course you are. Your mom just died.