Humanity
It's my opinion that Reiko becomes more and more human as the series goes on. She of course can't become fully human, as she is a parasite, but I think that she begins to think more like a human, identify with humans more, and at the end of her life it is evident that she feels human emotions.
At the beginning of the series when she was Ryoko Tamiya, she had a lot of parasite traits. She was instinctual, and she felt the need to eliminate threats, like A when he threatened her position as Tamiya, and even Shinichi after she quit being Tamiya and had no more reason to keep him alive. She still thought like a human, though, exercising judgment and questioning her existence. Why were the parasites there? Where did they come from? Where were they going? All of those questions were important to Reiko.
Human traits like self sacrifice, maternal instinct, individuality, and coming together as one in a society or community interested Reiko. All four are things Reiko eventually took on herself. Miki told Kusano in a parasite meeting that they wouldn't have what they had if it wasn't for Reiko; they wouldn't have banned together. She obviously saw the strength in numbers of humans (which she said the other parasites seemed to underestimate), and decided to use it for parasites.
Compared with humans, our behavior and thought patterns are fundamentally practical, simple, and clear. For that reason, I believed that we could form a group that would function smoothly...
Her individuality is pretty obvious. She was completely different from others in her species in so many ways. I think she wanted the other parasites to become more individualized as well. When Kusano decided she was a danger on his own and attacked her without Hirokawa or Goto knowing, Reiko was actually pleased.
This means that each of us have become more different, more unique, which pleases me immensely. This lynching is very impressive indeed.
Her self sacrifice and maternal instinct were proven when Kuramori kidnapped her baby and she went to save him, and when she chose to shield her son and take bullets instead of fighting back when she was being fired upon. The fact that she died for her son is the best proof of her self sacrifice. All of these things together show just how human-like Reiko thought.
Reiko seemed to not only have an understanding of humans, but I think she sympathized with them too. That is pretty evident by the low number of humans she ended up killing, but more than that, it's how even when she was in a position of danger herself by a human, she did everything she could not to kill them. Think about Kuramori who she tried to keep alive even after he found out about the parasites. Think about the guy on the train who wouldn't leave her alone. She did eventually end up killing both, but only when she had to. Any other parasite would have killed them sooner. Reiko also worked really hard to make humans and parasites coexist, and she tried to prove that parasites could exist without eating humans. Why would she take all of these steps if she didn't sympathize with them?
Reiko's human emotions are the biggest sign that she was changed by humans. She definitely cared about her child, if not loved him. Secondly, Kuramori fell out of his seat after getting really worked up near Reiko, and Reiko found herself laughing. She didn't know she could. She actually thought it was funny.
The other day, I stood in front of a mirror practicing how to laugh. That was so much fun.
Those were Reiko's last words right before she died. I think she died satisfied with how human she had become.