Bartosz Tiedemann's is a student at the Winden high school and Jonas Kahnwald's best friend. He is the only child of Regina and Aleksander Tiedemann.
In 2019, Regina owns and operates the Waldhotel Winden, and Aleksander is the director of the Winden Nuclear Power Plant. This places Bartosz among the wealthy elite in a town of mostly blue-collar power plant employees, but also isolates him from his classmates. Jonas is his only friend, and when Jonas is not around Bartosz spends his time alone smoking pot and playing video games. He seeks ways to prove himself and to fit in.
When Jonas was in treatment after his father Michael's suicide, Bartosz told their classmates he was on an exchange program in France, trying to protect him. At the same time, he became romantically involved with Martha Nielsen, who had been Jonas' girlfriend over the summer.
After the disappearance of Erik Obendorf, Bartosz led Jonas and the Nielsen siblings into the woods near the Winden Caves in the hopes of finding Erik's stash of drugs. When they arrive, Franziska Doppler has already claimed it, and he takes it from her forcefully, but the group is frightened by loud roaring coming from the cave and scatters. After that, he receives a call from Noah, and thinking him to be Erik's supplier, plans to meet him with Jonas later. Bartosz goes alone, however.
Bartosz is surprised one afternoon to meet his grandmother, Claudia Tiedemann, whom he had thought was dead. He later meets with Noah and as events have transpired as predicted since their first meeting, agrees to do as he says.
It is also possible that Bartosz is a descendent of Tronte Nielsen due to the later having an affair with Bartosz's grandmother Claudia.