Kusano
Kusano was in Hirokawa's political cabal of parasites with Reiko and Goto. He seemed intellectual like Reiko, but he had a different way of thinking. He was often on the opposite side of Reiko, questioning her position on issues, particularly when they related to Shinichi. They both saw Shinichi as strong. Reiko wanted him left alive so that she could study why. Kusano wanted him dead.
Why do our opinions differ? He's our enemy! He's dangerous!
Hirokawa sided with Reiko, but Kusano was persistent, and when the parasite death count went up because of Shinichi, Reiko had no choice but to consent to his demise. An experiment of Reiko's, Gogo and Miki, which were five parasites that shared one body, was put in charge of killing Shinichi. Kusano and his crew were in charge of killing a detective that had found out about the parasites' “safe houses” after investigating Shinichi for Reiko. Both failed. Kusano's crew killed the detective's family instead.
Reiko told Kusano that killing the detective's family showed a complete lack of understanding of human nature. He admitted to not understanding humans at all. I think that was the biggest difference between Kusano and Reiko. Reiko was interested in humanity and saw humans as more than obstacles and food. Kusano saw them as just that, and perceived them to be weak. Reiko realized their capabilities and strength in numbers. She also understood that humans outnumbered parasites.
I think Kusano began to think Reiko was becoming more human in nature through her studies, and I don't think he approved of her research. I think Kusano's way of thinking was more parasite than human -- he wanted to eliminate threats, while Reiko's was definitely more human; she wanted to understand her threats. He started noticing her changes, and I think he attributed it to Reiko's baby.
Tamura-san, you are raising a human child. Is it changing you? Perhaps it is time you disposed of it.
She refused to get rid of her baby, though, and Kusano began to look at her as something more than just a parasite with different views. He began to look at her as a danger.
Are you really one of us? You see things a little differently.
Kusano's natural way of handling threats led him to make an attempt at Reiko's life. He and two other parasites decided to gang up on Reiko without Hirokawa's knowledge. Her unique way of fighting proved too much for Kusano to handle, however, and she took out all three parasites. Before Kusano died, I think Reiko decided to mock him. It's just text on paper, so it's hard to tell, but when put with what she said to him when she first found out he wanted to kill her and he allowed her to ask questions first, it makes sense. Here, look:
Kusano-san, to hear such human-like words from you... I'm touched.
Have you ever thought like humans do?
I think Reiko knew that Kusano's issue with her was how in touch with humanity she was.