She has cancer.
In 1986, teenaged Regina was something of a quiet, anxious loner, who cut herself. Her mother, the confident new director of the Winden Nuclear Power Plant, criticizes her messy appearance and inattention. Her father is never mentioned. (Although believed to be Tronte S1 Ep:6 Lies)
She was a target of bullies at the high school; and once Ulrich Nielsen and his girlfriend Katharina Nielsen tied her up in the forest as a prank; she was found by Ulrich's younger brother, Mads Nielsen, who escorted her out of the forest. She blames Ulrich for Mads' disappearance.
Later, Hannah Kahnwald falsely reported Ulrich to the Winden Police, accusing him of raping Katharina. Hannah then told Katharina that it was Regina who had made the statement, prompting Katharina and Ulrich to attack her when they encountered one another in the forest. A mysterious teenage boy pulls a gun on them, and Regina offers to help him. He gives his name as Aleksander Köhler.
Regina sends Aleksander to the power plant to see if her mother, Claudia, the newly appointed director, will give him a job. Aleksander gets the job and marries Regina Tiedemann. In 2019, Aleksander becomes the director of the powerplant.
In 2019, Regina receives a medical report indicating that she has been diagnosed with late-stage breast cancer. She also gets into a late conflict with Katharina.