Appearance
Just like Reiko has two names in the series, the character has two major looks. Reiko's appearance is important because it drastically changes when she quits being Ryoko Tamiya. The two versions of Reiko are incredibly similar and very different at the same time. I'll analyze Reiko's first look as Ryoko Tamiya before looking at the Reiko we see for most of the series.
Ryoko Tamiya is a beautiful woman. The boys in Shinichi's school think she's hot, and she even gets attention from a guy on a train. As you can see, Reiko put a lot of care in her appearance as Tamiya. Her hair is done, it looks like she is wearing makeup, and she is even wearing jewelry.
Some time passes after Reiko quits being Tamiya before we see Reiko again. This is the Reiko we see for the rest of the series. She is still pretty in her own way. You can tell the two women are the same from the eyes and face shape, and they wear similar clothing, but that's about it. In the second look, Reiko's hair is flatter, she's not visibly wearing any makeup, and she wears no jewelry. Her choices in clothing are about the same. If I had to guess, I would say that Reiko continues wearing Tamiya's clothes after she quits being Tamiya. It's practical because the clothes are already there, and her comfortable appearance in all other areas contradict her clothing choices. She doesn't wear make up and wears her hair flatter, but she still wears skirts and heels. To me, that's the most logical explanation.
It's obvious Reiko doesn't spend as much time on her looks anymore. She's a lot more active in her observation and experiments by this time, and that's probably why she's more carefree with her looks. Reiko cares more about her studies and the future of the parasites as a whole than about her own needs, and to me, this look seems to fit the new Reiko more. She doesn't want to stand out in society like she stood out when she was Tamiya. She looks a lot plainer here, but she blends in more.