Mother
Reiko gave birth to a human boy that she decided to raise. She is a biological mother; at least her host body is. Was she a real mother in the traditional sense of the word, though? Although she was unconventional, I think Reiko was a mother to her baby, and a good one at that.
Reiko's baby was conceived accidentally as she was experimenting with sex. Her baby was the result of one of many experiments Reiko conducted trying to learn more about her species. She decided to bear it for experimental purposes. She justified it saying that if her baby wasn't useful in experiments, she could always eat it. I think the idea of a baby growing inside her fascinated Reiko. She was producing something that was of a different species, and I'm sure like everything else she wanted left alive (like Shinichi), she thought it would be useful in learning where the Parasites came from and what was possible for them in the future.
I think an event that happened while Reiko was still Tamiya interested her, too. Tamiya's mother visited Reiko after she heard she quit the school. Reiko was trying to brush her off and leave, but the woman wanted her to stop and look at her. When Reiko did, the woman asked her what happened to her daughter, immediately realizing that Reiko was not her. Reiko killed the woman because she was a danger to her identity. The woman intrigued Reiko, though. She wondered how Tamiya's mother knew she wasn't Tamiya. She didn't look or sound any different. Because of this, I think part of the reason Reiko decided to keep her baby was to study maternal instinct.
So she bore her baby. It showed up a little later in the series. I don't think she ever named her son, but that would only make sense. Parasites are indifferent to names; names simply don't matter to them. It didn't mean she loved her baby any less.
Reiko often carried her baby around by the collar of his clothing rather than with the care people often carry babies with. It made outsiders look twice and question whether she was a good mother. Upon closer examination, people were able to see that she had a way with her son, able to make him stop crying by just putting her hand on his head and saying “Shush.”
I have it trained well, don't I?
I don't think she carried him around like that because she didn't care about him. I think she was just inexperienced; she didn't know how to handle a baby. Reiko had to study her facial expressions to make sure she could convey emotions properly, and those are pretty basic. Reiko had to study everything. It just goes to show that nothing conventionally human came naturally to Reiko, least of all childcare. Reiko probably knew she wouldn't know how to raise a child, and I think that's why she decided to get a human to babysit her son for her -- to help raise him.
A couple of times throughout the series, Reiko threatens to use her baby as a shield if Shinichi and Migi choose to fight her. It can be argued that Reiko doesn't love her son because she's able to dispose of him so easily, but I think it's just for show. I think she knows Shinichi won't fight her if it means a human baby dies. Furthermore, later in the series a detective kidnaps her son as an act of revenge for his family dying at the expense of parasites, and Reiko meets him to save it. She can't let harm come to her son.
Shinichi and Migi were nearby when Reiko went to meet the detective. The detective held her baby over the railing of a bridge about to drop it, and Reiko emitted a signal Migi wasn't able to recognize. He said it was unlike any signal sent by other parasites.
What emotion is this?
I think the signal and feeling that Migi was picking up on was love. It was unlike anything any other parasite was able to experience, but I think by having a human child and raising it, Reiko did fall in love with her son. She killed the detective to save her baby. As the detective was dying, he told her that he was just joking and would never kill a human child, but that he didn't think a monster like Reiko would even care. She says neither did she.
I think by that point, Reiko realized how important her son was to her. I think that's why later when the cops circled her and began shooting her, all she could think about was how to get her baby to Shinichi. She didn't even fight back even though she could have, and I think the reason she didn't was to save her son.
She hands the baby to Shinichi before she dies and tells him that it was never experimented on and to make sure it is raised well. That was her final wish before she died. She told him that she had gone through life asking questions, and every question that she found an answer to led to another question. She said that day she found an answer to another question. I think it was the bond between a mother and a child that she questioned when Tamiya's mother knew instantly she wasn't her daughter.
I think most mothers you meet will tell you that they would die for their children. Reiko did.