Peter's family connections are somewhat mysterious. He is the son of Helge Doppler, but his mother is not mentioned. Although Helge is shown as residing in Winden in 1953 and 1986, Peter mentions that he arrived in Winden in 1987.
Peter's marriage to Charlotte is troubled, although they try to hide their unhappiness from the others, upsetting Franziska. They have not shared a bed for some time. Peter is known regularly to have visited a prostitute, Benni, although at the start of Season 1 he has not visited in a year.
On the night of Mikkel Nielsen's disappearance, Peter drives to the truck stop where Benni works, but stops a distance away and does not get out of his car. Benni sees him drive away.
Peter heads instead to the cabin in the forest and descends into the bunker, where he sits reciting the Serenity Prayer to himself. Suddenly, a wormhole/portal appears in front of him, and the body of a young boy drop out. Startled at first, he attempts CPR on the boy, but finding it ineffective, searches for and finds the boy's school ID card. It is Mads Nielsen, who disappeared in 1986.
Peter is reserved, and suffers silently, not showing his emotions even with family members. He has many internal conflicts. His sexual desires conflicts with his responsibilities to his family and his marriage. He also considers confessing to Charlotte something (presumably that the dead boy found in the woods is indeed Mads Nielsen), but he always falls short of doing it.