In the Völsunga saga, Sigurd’s foster father and mentor is Regin.
Regin is a dwarf and the son of Hreidmar, whose family becomes entangled in the curse attached to Andvari’s gold. He raises Sigurd and teaches him skills appropriate to a legendary hero, including knowledge of weapons and warfare. Regin also tells Sigurd about Fafnir, his brother, who has become a dragon while guarding the treasure.
Regin persuades Sigurd to kill Fafnir and helps prepare the sword Gram for the task. After the dragon’s death, however, Regin plots to take the treasure and is killed by Sigurd. His role combines that of foster parent, instructor, storyteller, and eventual betrayer.
Regin is not Fafnir: Fafnir is the dragon Sigurd fights, while Regin is the craftsman who guides him toward that encounter.