Character
Reiko Tamura begins the series as Ryoko Tamiya, a 24 year old substitute math teacher in Shinichi's school. Reiko is a parasite; unlike Migi, she succeeded in taking over her host's brain. For a while she kept up the life her host body had and blended in with society. Circumstances had her leave that life, however, and she took on the name Reiko Tamura to start a new identity for herself.
Reiko is extremely scientific. She is fascinated by Shinichi because he's an anomaly, and she is hugely interested in humanity. Reiko also experiments on parasites, and she created Miki and Goto, super parasites with three and five parasites in one body.
Reiko is highly respected by other parasites. They often rely on her advice in making decisions, and she is a big part of a man named Hirokawa's secret group that was formed to help parasites better coexist with humans, partly by setting up “safe houses” to kill and eat victims in private instead of just leaving mutilated corpses lying around for humans to see and fear.
Reiko is the parasite that knows humans best. She is often seen giving advice to other parasites on how to act more human. I think she knows humans so well partly because she gave birth to a human child. Her pregnancy and interaction with her baby are major components of the series.
What I believe to be most interesting about Reiko is how Shinichi perceives her. She's a parasite that kills and eats humans, but she takes actions to keep Shinichi and Migi alive even though they are killing her species. So is she good or evil? I guess you'll have to read the series to find out.