A
A is the father of Reiko's baby. One would assume by the other parasites Reiko interacted with that A was someone smart, important, and interesting. A was none of those things, though. In fact, it's hard to see what Reiko admired about A in the first place. She did admire him to an extent, however, because she did think enough about him to bring him to meet Shinichi. And she did think enough about him to conduct experiments with, as that was what sex was to them; they weren't in love and didn't have any kind of romantic relationship. I don't think parasites had meaningful relationships like that.
The narrator stated that the parasites were still very young and that Reiko had made a mistake by allowing A and Shinichi to meet. I think Reiko thought A was better than he actually was and would be able to exercise judgment over instinct. She quickly learned that wasn't the case. A came prepared to fight Shinichi and Reiko had to tell him and Shinichi to stop, that they gain nothing by fighting.
A and Reiko are essentially opposites. Reiko is a thinker. A is instinctual. Reiko is interested in what they are, where they came from, and where they're going. Those questions bore A.
Learning what I am changes nothing. I eat when I'm hungry. I fight when I'm threatened.
Reiko wants to study Shinichi because he‘s different. A wants to kill Shinichi because he's different.
I guess A couldn't help but feel threatened by Shinichi. I think Reiko was able to make him control his urges to an extent, as he didn't go crazy and attack Shinichi in the coffee shop where they met. His urges and inclination to kill what threatened him got the better of him, though, because he came to Reiko and Shinichi's school to kill Shinichi. Reiko was teaching at the time. She and Shinichi both sensed A had arrived at once, and Reiko yelled, “Idiot!” She gave her class free study and left the room.
Shinichi and Migi were too much for A to handle. They almost killed him and left him to die. The last we see of A is him searching for Reiko, hoping to join her in her body. He dies in a lab explosion, though. It's left open, but the most logical conclusion is that Reiko killed him. Her identity as human was important to her, and A threatened it with no concern by showing up to her school. He also was going to kill Shinichi, who she wanted to study more. Either or both of those could have been reasons for her killing A, but I personally think it was because she endangered his identity. The narrator said that the parasites were still relatively young at that time, remember? Even Reiko had to have acted on instincts once in a while. She, like A, killed what threatened her. In her case it was A.