Aleksander Tiedemann (previously Aleksander Köhler, originally Boris Niewald formerly Chuck) is the director of the Winden Nuclear Power Plant. He is married to Regina, with whom he has a son, Bartosz.
Boris Niewald arrived in Winden in 1986, apparently on the run. He encountered Ulrich Nielsen and Katharina beating up Regina Tiedemann and scared them off by brandishing a gun. He gives Regina the name Aleksander, but Hannah Kahnwald later sees him burying a bag in the forest containing the gun and a passport with his photo and the name Boris Niewald issued in Giessen, swapping it for another issued in Marburg for an Alexander Köhler. Dark Season 1 Episode 9: "Everything Is Now"
By 2019, Aleksander is married to Regina and director of the power plant, a position once held by his mother-in-law Claudia. Hannah is his massage therapist.
Ulrich finds a locked door in the cave that seemingly leads to the nearby nuclear power plant. He tells Charlotte to request a search warrant for the power plant. Aleksander blocks this and when Ulrich requests access he refuses him entry, citing security regulations. He also has things taken away before the police can find them. Dark Season 1 Episode 2: "Lies"
Hannah, having taken Aleksander's bag to their massage appointment, blackmails him with it. She demands no money, only the destruction of Ulrich's life. In turn, Aleksander calls his friend in the Winden Police, Torben Wöller, and tells him to dig up dirt on Ulrich. He also mentions that they will soon be able to return the barrels of waste, which had been stored on a truck. Dark Season 1 Episode 9: "Everything Is Now"