Peter Jürgens is a sexual offender currently a research subject of Dr. Rudy Gillen during the length of the series. His testimonies to Dr. Gillen proved Johan Liebert's existence as well as his notoriety. It was also seen that he is taken by Johan's extreme familiarity on him that he actually killed himself at the sight of Johan's photo.
Involvement with Dr. Gillen's research
Jürgens became one of Dr. Gillen's research subjects due to his involvement with murders that is sexual in nature. His victims were mostly high school girl from ages 16-18. His twelfth murder, however, was odd considering how he picked his victims. He confessed that his murdering of the 52-year-old woman was done per the request of a friend. What Dr. Gillen recorded when he was visited by Dr. Tenma was made into a research material. He made Jürgens listen to it and was asked what he thought about it. Jürgens said he could relate to it, and that the "monster" exists. Dr. Gillen took it as a metaphor and he agreed that monster exists, and that it dwells in him and in everyone else. Jürgens did not buy it, saying "Stop your academic nonsense". He insisted monsters really exist in this world. Jürgens then glared at Dr. Gillen and tried to strangle him. When Dr. Gillen reached for the alert button Jürgens snatched it and asked him if he is scared. He also instructed him to visit the basement of a certain Kempf Mansion. Jürgens said he'll understand what he's saying by going there.
When he found the stairs that leads to the basement, he started recording his journey down. He recorded some bits of information Jürgens told him, like Jürgens' visit to the same basement before killing the old woman sleeping at the second floor. At first Dr. Gillen only saw an ordinary storage room full of pictures of Frau Kempf and with a bespectacled boy. He also saw the large Parisian doll Jürgens describe. He was about to dismiss Jürgens' claim that this place will make him believe that monsters exist and that he did this murder to mess the trail the detectives are following. Dr. Gillen then remembers some statement of Jürgens that made him stop for a while and load another cassette tape. He heard from the tape that he used to plead help from that long haired Parisian doll from their storage room but it would just look at her and grin. That storage room was full of pictures too that anywhere he looked, his mother's face was there. Dr. Gillen then noticed the irregularity in the room: the boy in thick glasses were, for some reasons, replaced by another face, the face of Jürgens. He realized Jürgens could not have known Frau Kempf in his childhood, and that the face originally captured there was in fact, the face of the child once in the foster care of Frau Kempf.
Jürgens just did what the letters from 'friend' sent him. The letters were written as though 'friend' knew him alot, and he was angry about it. He got letters every week after that first anonymous letter. It said it was from a demon. Jürgens, in the process, enjoyed receiving the letters, because it sounded like they were really friends. One day, the letter instructed him to visit Frau Kempf's basement. Jürgens saw Frau Kempf's basement exactly like their childhood storage room where he gets punished. That basement confused him, because his pictures were placed beside Frau Kempf. This triggered Jürgens' desire to kill her.
This confession heightened Dr. Gillen's understanding of what was going on. He searched Frau Kempf's room and found a stack of letters. The letters contained the messages Dr. Tenma showed him during his visit. This made him believe that Jürgens's 'friend' and Johan really exist.
Dr. Gillen acquired a photo of Johan from Dr. Reichwein and brought it to Jürgens. Jürgens tells Gillen that he met Johan at a park and he used a pen to commit suicide.
"There are things out there that would turn your blood into ice, doctor"
"Yes, I was punished every day as a child, by my mother. Whenever I was hit, I would always go down to the basement… and down there was a huge doll my mother loved… a long-haired… female doll. When I was being hit, I pleaded to that doll for help… but it just sat, grinning and watching in the darkness, as my mother beat me. I don’t even want to remember… that place… And in that storage room were many many pictures of my mother. Everywhere I looked… mother… mother… mother… always, always watching over me!"
"Come with us." - His last words
Jürgens seems to be based on real-life serial killer, Edmund Kemper, for many reasons: Both are very similiar in appearance, their large size for example. Both initially targeted young girls but later killed elderly. Both threatened their interviewers. The surname of his twelfth and most important victim, Kemp, is similiar to Kemper.