Volsunga Saga and Þiðrekssaga
The son of Sigmund and Hiordis, it is said he was born after his father was slain in the battle with Lygni. His mother was later remarried to Elf, the Viking king, who treated him as if he were his own son.
He was tutored in the martial arts by Regin, and when he reached maturity, was given the horse, Grani, who was descended from the stock of Sleipnir. Regin would later tell Siegfried the story of Fafnir, of how he slew their father for a great sum of gold, and took the form of a monstrous serpent in order to guard it. Siegfried agreed to slay the beast, on the condition that Regin forged for him an unbreakable sword. Two of the swords forged by Regin broke upon handling, and he finally agreed to forge a sword from the fragments of the Balmung, which Hiordis had saved.
On his voyage back to Hunlaland Siegfried met a man walking on the surface of the sea, whose name was Feng, though he was actually Odin in disguise, and who taught him how to perceive and recognize auspicious signs. He later slew Lygni and reclaimed the throne of his father.
Slaying of Fafnir
When it was finally time to slay Fafnir, Odin appeared before him once more, advising Siegfried to dig a trench below the route on which he would travel each day. From within the safety of the trench he was able to pierce the dragon's heart as it flew overhead. Regin advised Siegfried to remove and cook the dragons heart, and when he had done as advised, he gained the ability to decipher the language of the birds, and they told him of Regin's malevolent ambitions. So Siegfried slew Regin and bathed in the blood of Fafnir, which made his body invulnerable, except for a single spot on which a leaf had fallen.
The birds then told him of Brunhild, a Valkyrie who had been banished from Valhalla for disobedience, who lay sleeping enclosed in a wall of flame at the Hindarfiall. Siegfried easily broke through the flames with his steed, Grani, and took Brunhild as his bride.
However Siegfried later fell in love with Gudrun, daughter of King Giuki. Later, when Gunnar succeeded his father as king, he fell in love with Brunhild and, with the assistance of Siegfried and a series of tricks, took her hand in marriage. But Brunhild soon grew bitter with the tricks played on her by Siegfried, who she still loved. When she had quarreled with Gudrun over Siegfried she asked her new husband to kill him. But Gunnar, as well as his brother, Hogni, was bound by a blood-oath to Siegfried. Nevertheless, Guttorm, who had long held a grudge, took up the request, stabbing Siegfried in the back with his spear. With his last ounce of strength Siegfried took up his shield and bludgeoned Guttorn to death before finally succumbing to his injuries.
Siegfried is a recurring figure in the Megami Tensei franchise, often featured as a demon of the Fury or Hero clan, or in the case of the Persona series, a member of the Strength Arcana.
Devil Summoner: Soul Hackers
Siegfried appears in Devil Summoner: Soul Hackers as a demon of the Hero clan. His design notably incorporates elements from the Volsunga Saga, including the mighty great sword, Balmung. Siegfried's skin is noticeably bronzed with the exception of a spot on his right shoulder, where a leaf had fallen during his famous bloodbath.
Shin Megami Tensei: Devil Summoner - Raidou Kuzunoha vs. the Soulless Army/King Abaddon
Siegfried was later featured in Shin Megami Tensei: Devil Summoner - Raidou Kuzunoha vs. the Soulless Army, and it's 2009 sequel, as a demon of the Fury clan.
Shin Megami Tensei: Persona 3
Shin Megami Tensei: Persona 3 introduced Siegfried as a member of the Strength Arcana, available upon completion of it's respective Social Link. The fusion spell "Scarlet Havoc" can be used when both Siegfried and Ares are present.
In Shin Megami Tensei: Persona 3 FES Siegfried can be combined with a "Nihil Longsword" to form the mythical Balmung.
Shin Megami Tensei: Persona 4
Siegfried reprises his role as a Persona of the Strength Arcana in Shin Megami Tensei: Persona 4. He is also among the Personae called upon by Margaret at Heaven's Paradise.