Beginning
Reiko Tamura makes her introduction as Ryoko Tamiya, a substitute math teacher in Shinichi's school. She's introduced as 24 years of age and single.
Reiko took on the career and life of Tamiya, basically, choosing to blend in with society and hide her identity as a parasite. I can't help but wonder what benefit Reiko had in trying to be Tamiya. It wasn't to have easy access to food, as feeding became more difficult for her. Like Migi eventually picked up on, because Reiko wanted to blend into human society, she wasn't going to feed on anyone directly related to her life. She'd have to outstretch, and hide to feed. I think she probably took on the teaching position and Tamiya's life to study humans. We learn later in the series how scientific and curious Reiko is. It would definitely make sense she'd want to study humans.
When Reiko was being introduced on stage as Ryoko Tamiya, both Reiko and Migi were able to sense each other right away and made eye contact. Even other students around Shinichi in the crowd could see that Tamiya was looking in their direction. This began Reiko and Shinichi's odd relationship, or rather, Reiko's fascination with Shinichi. Unlike previous parasites Shinichi and Migi had encountered who saw that Shinichi and Migi were different and tried to kill them, Reiko made no attempt to harm Shinichi. She saw that he was different and it interested her. She wanted to learn more about him. She taught her math classes with competency, (We learn from Migi's quick learning rate that the parasites are able to adapt quickly.) and she treated Shinichi just like another one of her students, at least in public. In private she told him that she was interested in him because he was an anomaly and would not harm him. She let him know of her capabilities, however, should he decide to do anything irrational to her.
I wish to protect my identity as a human, so I don't plan to do anything dramatic. But if I changed my mind... one class would take three seconds to kill.
Reiko definitely didn't have any intent to kill anyone from her day to day life as Tamiya. She only killed one man as Tamiya, and I believe it was in self defense. I think she did everything she could not to kill the man. He groped her on a train, and she called him out on it. Embarrassed, he began making a big scene on the train. Reiko told him that she forgave him and to shut up, but he continued yelling at her. She punched him in the face and then picked him up with one hand by his neck and threw him out of the train onto the platform. It wasn't until he decided to follow her home from a coffee shop that she killed the man. He was worried about how people would see him on that train after what had happened to him and decided to “make her pay”; he had a metal pipe that he was going to hit her with. It was obvious that the man was not going to leave her alone, and that's why I think she killed him. I think she hated that she had to needlessly kill someone too, because before she did it she said, "I'm not hungry now."
Reiko did everything she could to protect her identity. She even killed A, another parasite and the father of her baby, after he came into the school to kill Shinichi, endangering her identity. Shortly after that, though, her time as Tamiya was cut short by signs of her pregnancy. The other faculty members expressed worry about the kind of message she'd be giving to her students as a single pregnant woman who wouldn't even provide the name of her baby's father.
I never thought it would end this way. I don't want this kind of attention. I quit being Tamiya Ryoko.
And just like that she was done. She formed a new identity for herself and started going by Reiko Tamura. Migi later said that parasites don't care much for names, so her new name would probably be close to Ryoko Tamiya. Her new name definitely has a lot of the same sounds. I really like the fact that she kept a piece of her beginning in her identity.