Fafner

One of the Giants, who built Valhalla for Wotan. He is more calculating then his brother Fasolt. When the God reneged on the deal, Fafner and his brother Fasolt kidnapped Wotan's sister-in-law Freia. Ultimately, they agree to payment in the form of the treasure of the dwarf Alberich, which includes the Tarnhelm and the Ring, only to be murdered by the covetous Fafner. Transformed into a dragon while hoarding his treasure, he is ultimately killed by Siegfried, who is guided there by Mime, who originally crafted the Tarnhelm and wanted to reclaim both objects of power.

Das Rheingold
When Wotan reneges on his payment for Valhalla, Fafner and his brother kidnap Wotan's sister-in-law Freia. While Fasolt wants her for her beauty, Fafner wants her as she tends the Golden apples which keep the Gods young and he hopes to deprive the Gods of their power. He convinces Fasolt to agree to accept the treasure instead of Freia. Wotan eventually concedes the ring, Tarnhelm and treasure to the Giants, who argue over the division. When Fasolt takes the ring he is immediately murdered by Fafner.

Die Walküre
No appearance. However he is mentioned when Brunnhilde tells Sieglinde to flee to the woods where Fafner lives so she will be safe from Wotan.

Siegfried
In the time since Das Rheinhold, Fafner has retreated to the woods, and as a result of Alberich's Curse transformed into a dragon, jealously guarding his treasure, while Alberich watches his cave, which is called Neidhöhle. Siegfried confronts the dragon, challenging Fafner to "teach him fear," before killing him in battle. Siegfried gains the ring and the Tarnhelm, and by drinking Fafner's blood gains the ability to understand animals, and to read the thoughts of humans.

Gotterdämerung
No appearance, but mentioned in passing when Siegfried recounts his adventures.