The brother-sister relationship between the Witch, Freya Mikaelson and the Original Vampire, Finn Mikaelson.
Freya and Finn are the two eldest children of the Mikaelson family. After Freya was taken by their aunt Dahlia, Finn though for centuries that his sister was dead, as she was not an immortal. They finally reunited in The Devil is Damned, and ever since, their relationship has been incredibly strong.