*might contain spoilers*
An extremely intelligent, ruthless Parasite and the series' main antagonist alongside Takeshi Hirokawa. She originally infected and took on the identity of a woman named Ryoko Tamiya, one of Shinichi's high school teachers. Unlike most of her kind, she is motivated by scientific inclinations with a drive to understand her kind's biology, origin, and purpose. She learned that she is pregnant with a normal human infant as a consequence of being impregnated by Mr. A, deeming it interesting while forced to quit to avoid unwanted attention resulting from it.
Though she intended to kill Shinichi soon after, she lets him and Migi live unmolested largely because she finds them an interesting anomaly worthy of study. After killing off Ryoko's parents when they saw through her, the Parasite alters her face and creates the identity of "Reiko Tamura" while aligning herself with Takeshi Hirokawa. She eventually gives birth to Mr. A's baby and later kills three of her fellow Parasites when they deemed her actions with a hired detective a threat to their plans. It was after Reiko killed the maddened detective to save her child that she allowed the police to brutally gun her down in a park instead of trying to resist or escape. The baby survives because of her protection and decision not to fight back against or run away from the police.
Tamura Reiko might be termed as the main antagonist of the series but she is one character who gives meaning to everything happening in Shinichi's life. She was parasite, but she understood the fact that, parasites are afterall a week species who needs a host to survive and she wanted to co-exist peacfully with humans in their society.