The character of Hareth is taken from two main sources and two characters who actually have nothing in common but the role of lovers for Thidrek or a Thidrek-like figure, and a slight similarity in the names. That alone, however, was enough to fire my imagination...

The first source is the Thidrekssaga. Thidrek, that is Dietrich von Bern, marries Häredh (cap. 290) (or Herrat, Herrath, Herrad, Herath, Herad), niece of Queen Helche, the wife of Atli/Attila, when Helche dies entrusting her as a wife to Thidrek.

We learn more about "this" Herrat in a related source, Adventure 22 and Adventure 23 of the Nibelungenlied. She is an "exiled maid" (Shumway translation, linked above), or "stranger maid" (Needler translation - unfortunately I don't have the original yet), daughter of Nentwin and betrothed to Dietrich, who attends to Queen Krimhild while grieving for dead Queen Helche. Today Herrad is still a fairly common German female name.

The Thidrekssaga always mentions Herrat with deep tenderness on Dietrich's part. The hero leaves Atli's palace alone, with the only company of his wife and his faithful master-at-arms Hildebrand. He defends her from bandits, always helps her dismount from the horse, sleeps by her side in the forest during their exile. Many years later, when Herrat dies, the old king finds no more pleasure in life, if not when hunting, and he spends all his time with horse and hounds, thus preparing his final, spectacular disappearance on an otherwordly black horse, a theme that will be tied also to the historical Theodoric the Great.

The second main source is Saxo Gramaticus, who speaks of the legendary hero Hadding. According to Viktor Rydberg, the figure of Dietrich von Bern is directly descended from the golden-haired Hadding. Now, Hadding's first love is his foster-mother, a warrioress named Hardgrep, able to shape-change into a giantess. She follows Hadding in his quest to avenge his father, dressed as a man, but immediately she meddles with magic and the Otherworld and she is killed by a spirit in the form of a giant black hand.

It's not very philological (I haven't made a study of what they mean) but I simply fused the names Häredh and Hardgrep to make Hareth; in the same way I chose the parts of the stories I liked most. The whole "foster-mother" angle was a bit icky, so I turned Hareth into Thidrek's mistress-at-arms when he was at Atli's court as a kid. Suppose she was 14, already a woman, and he was 11, still a child. Ten years later, the difference is less important.

I might take yet another story angle from Saxo and write a storyline where I kill Hareth off... so you may not want to get too fond of her!