Season 1: Bartosz is the only child of Regina and Aleksander, Jonas's best friend, and Martha's boyfriend (which is complicated, because he kind of steals her from Jonas while Jonas is off getting treatment following his father's death). He's there when Mikkel disappears and later is met by his grandmother, Claudia, who he had thought was dead, and then Noah, who tells him about time travel and shares what Claudia and Jonas are up to in the future.
Season 2: In this season, Bartosz teaches Magnus, Franziska, and Martha about time travel by taking them to the '80s as proof. Upon returning to 2020 on the day of the apocalypse, the time machine is taken from Bartosz and ends up in the hands of Katharina, but older Jonas ultimately saves him, Magnus, and Franziska from the apocalypse by travelling with them at the last second.
Season 3:
- Adam's World: After traveling with Jonas, Magnus, and Franziska to the Tannhaus workshop in 1888, it's clear that he is the friend most in tune with the changes Jonas begins going through on his way to becoming Adam. Frustrated with Jonas for not being able to get them back to the future, he storms away from the property, where he meets Silja in the woods. The pair eventually marry and go on to have two kids: a son, Hanno, and a daughter, Agnes.
- Eva's World: In Eva's world, Bartosz learns about his father's true identity, then moments later is told by Martha that his father was involved in covering up the dark matter at the power plant that causes the apocalypse. He tries to help Martha to stop the apocalypse from happening, but they're intercepted on the way by old Magnus and Franszika, who travel away with Martha. Then, his older self appears there on Eva's orders to send him to stop Martha from saving Jonas from his apocalypse in a parallel version of this reality (a true mindf*ck).
- Origin World: Regina is seen without Aleksander in the origin world, and with Katharina as a friend, rather than an enemy who bullies her in the woods, it's likely that Regina never came in contact with Aleksander — whatever became of him — so if that's the case, Bartosz can't exist.