Michael Kahnwald was really Mikkel Nielsen, who traveled through time from 2019 to 1986 and grew up with adoptive mother Ines Kahnwald. Thus, Jonas's biological paternal grandparents are the Nielsens: Ulrich and Katharina, making Martha and Magnus his biological aunt and uncle.
In the days following Mikkel's disappearance, Jonas's true purpose is revealed: He is a time traveler who isn't aware of how significant a role he will play in the overall course of events.
As an adult, he travels through time with a complicated agenda. He is working tirelessly to undo some of the unfortunate events in Winden that he has helped to create. While he suspects he can't actually change the course of time, he has not given up hope. For a long time he works alongside Claudia Tiedemann in her fight against the Sic Mundus cult and their leader Adam.
As an old man, Jonas turns into Adam and develops the goal to maintain the cycle of time. He forms Sic Mundus and becomes the manipulative orchestrator of events.