Kuramori
Kuramori was a private detective that Reiko hired to watch Shinichi. He was human, but that's the reason Reiko hired him. She probably wanted him to be able to watch Shinichi without Shinichi or Migi knowing. If a parasite did it, Migi would pick up its brainwaves and they would know they were being watched.
She wanted to know if anything was different about Shinichi, trying to find the source of his strength. Kuramori found nothing unusual at first, but Reiko had him keep watching Shinichi until he did. She paid four humans to attack Shinichi, probably to see how he'd deal with them. Kuramori saw Shinichi take on all four guys and win. He told Reiko he never saw a person move that fast. She asked him if he noticed anything inhuman, and that's when she made a mistake with Kuramori. She asked him if he noticed anything strange about Shinichi's right hand. Of course at that time, he didn't.
Kuramori kept watch over Shinichi and he did eventually see Migi. He even snapped a photo. Migi destroyed the camera, and Kuramori escaped with his life. His life wouldn't be the same though. He had seen a monster, and it changed his view of the world.
When he told Reiko about seeing Migi, she played stupid. She said maybe he needed some help, and that the story was absurd.
I'll pay you for the work you've done, but it's time we stopped this.
I think Reiko wanted to learn more about Shinichi's strength, but once the detective discovered Migi, things became dangerous for him, Shinichi, other parasites, and Reiko herself. Kuramori remembered that Reiko had asked about Shinichi's right hand, so he knew that she knew about him. She contemplated killing the detective, but for whatever reason decided against it. I think she really didn't want harm to come to Kuramori. By this time, she was thinking more like a human and was probably trying to live without killing or eating them.
Kuramori pretty much dug his own grave though. Curiosity overcame him, and from learning more information from Shinichi, he learned that Reiko was a parasite. He had one of his assistants follow her around which led his assistant to follow another parasite to a “safe house” where they fed. Kuramori's assistant was killed, but he was wearing a wireless talking to Kuramori who was on the other line. Once the parasites learned of it, Kuramori became a target. Reiko tried to deter them from him.
I think now... is not the time to be making waves.
Kuramori showed Shinichi the “dining room”, though, which led to another parasite being killed. Once the body was found, there was nothing Reiko could say or do to stop Shinichi or Kuramori from becoming parasite targets.
Kuramori was out when the parasites visited his home. When he came back, he found the bodies of his wife and child. He was put in protective custody which kept him safe from the parasites, but he escaped to avenge his wife and child.
Youko... Yumi... I will avenge you! Tamura Reiko!
To Kuramori, Reiko was the reason he even knew about the parasites. She was the reason his family was butchered. There was no way he could have known that she did what she could to keep him safe. He kidnapped her baby and left a note for her to meet him at a park in the evening. There, he talked about family.
...You don't really understand. You don't really know what it means to be a parent. Do you know how happy we were? Do you know how much I grieved at the loss? If I killed this child now, would you be sad? Or would you simply understand?
He said he didn't expect her to understand, but if she felt even a hundredth of his pain... and then Reiko killed him. He didn't even get to finish his sentence. He was lifting her baby over a bridge to throw it off, and she had to kill him to save her son. She waited and listened to him speak, and didn't even try to take him out until she saw that her baby might die. I think that shows she didn't want to kill the detective.
Kuramori told the cops what Reiko looked like before he died. I think he was shocked to learn that the baby was loved though. He wasn't a bad person and I'm sure he didn't want to put a parent through what he was going through. His last words were pretty interesting.
I'm glad I didn't kill the baby...