Martha is the biological daughter of Katharina Nielsen and Ulrich Nielsen. She is also the older sister of Mikkel and Jonas' auntie. At the start, we meet Martha who's very talented in drama and who's in a relationship with Bartosz Tiedemann. The awkward tension between her and Jonas shows that they dated before he was sent away. However, we later see that Martha and Jonas share a passionate kiss together. Your mama.
Martha is a confident and talented teenager who does have complications at home when her younger brother mysteriously vanishes. Martha maintains a very strong relationship with her brother Magnus after Mikkels disappearance.
Though Martha does appear quite often throughout the series, she doesn't really play a significant part.