Here’s my take on Siegfried (or Seyfried, or Sigurd for the original character of Nordic mythology, or even Sivard among the Danes), the hero of the Germanic epic called the Nibelungenlied. This medieval epic, although tinged with Christianity, was heavily influenced by the Völsunga saga and the Eddas, compiling older Norse myths. Siegfried also appears in the last two operas of Richard Wagner's lyric tetralogy "The Ring of the Nibelung", namely "Siegfried", and "The Twilight of the Gods". Naturally, the different versions of his story differ, but I'm going to tell them backwards, to the original Norse myth.
In Wagner's tetralogy, Siegfried is the son of the twins Siegmund and Sieglinde, themselves the fruits of the union between Wotan/Odin and a mortal ; which makes Siegfried a demigod. The couple having committed adultery (Sieglinde had been forcibly married to an enemy of Siegmund, previously) then incest, Fricka/Frigg, wife of Wotan and goddess of marriage, demands punishment from the one-eyed god. The latter hoped that their descendant could prevent the Twilight of the Gods, but resigned himself to ordering the Valkyrie Brünnhilde (who is also one of his daughters) to favor Siegmund's enemy, by withdrawing the magical force of his sword Nothung. However, moved by the situation of the desperate couple, Brünnhilde disobeys her father and restores her power to the sword; but Wotan suddenly appears, and breaks the magic weapon, condemning Siegmund. The terrified Valkyrie has just enough time to pick up Nothung's debris and carry off a fainted Sieglinde on her horse. Brünnhilde informs the poor demigoddess that she is pregnant, that her son will reforge Nothung, and that he will become the greatest of heroes ; Sieglinde then flees to a place where she and her child will be safe from the wrath of Wotan, leaving the Valkyrie to face her father, who condemns her to eternal sleep and to become human, not without protecting her from the first man to come with a fire barrier.
We will learn later that the young Siegfried was adopted by the nibelung/dwarf Mime (with whom he has conflicting relations), probably following the death in childbirth of Sieglinde. As a young man, Wotan's grandson is famous for his phenomenal physical strength, being able to capture a living bear on his own, and breaking all the swords that Mime forges for him. The nibelung ends up challenging his adopted son to go and face the terrible Fafner, who has become a dragon after seizing the cursed treasure of Alberich, fallen king of the nibelungen. Siegfried then reforges Nothung, and even breaks the anvil with his new weapon ; then he leaves to find Fafner's lair, followed by Mime, who hopes to be able to poison Siegfried to seize the treasure, once the dragon is defeated. The demigod ends up triumphing over the monster by piercing his chest, and Fafner warns him of the curse that weighs on his treasure. By tasting the blood of the dragon, the superman begins to understand the language of the birds. Precisely, a bird tells him about the treasure, including the magic helm called Tarnhelm (allowing the metamorphosis or the invisibility of its wearer), and the famous Ring. Siegfried, having understood Mime's plans, kills the dwarf, then seizes the two artifacts. Later, guided by the bird, the hero approaches the rocky peak surrounded by flames where Brünnhilde is asleep, and there meets her grandfather Wotan, traveling in disguise, with whom he ends up arguing. Much to the god's shock, Siegfried manages to break his magic spear with Nothung ; Wotan then fades away, allowing his grandson to break through the barrier of fire, and discover the beautiful sleeping former Valkyrie. First shaken, the demigod ends up waking her up with a kiss, and an absolute passion is born between him and the one he doesn't know is his aunt.
Many days later, Brünnhilde sends her lover on new adventures, but the latter entrusts her with the Ring as a token of loyalty, before setting off on horseback. Siegfried's wanderings take him to the court of King Gunther, on the banks of the Rhine. But Gunther covets the former Valkyrie, and his sister Gutrune the demigod ; Hagen, their sinister half-brother (fruit of the union between Queen Grimhilde and the dwarf Alberich) leads Gutrune to make Siegfried drink a potion of oblivion. The hero then forgets Brünnhilde, and falls in love with Gunther's sister, whose hand he asks. They end up making a pact : Siegfried will be able to marry Gutrune if he manages to bring Brünnhilde back to the king, by impersonating him thanks to the Tarnhelm. The two depart for the former Valkyrie's Rock, while Hagen rejoices at the prospect of reclaiming the Ring. In the guise of King Gunther, Siegfried abducts Brünnhilde and seizes the Ring, before arranging for her return to the palace. When Siegfried's lover arrives at the king's court, she finds that the demi-god wears the Ring, and everyone is confused. And no more space to finish the story...