Siegfried (ジークフリード?, or ジークフリート?) is a demon in the series.
Siegfried is the dragon-slaying hero of the medieval German epic Das Nibelungenlied, most widely known due to its adaptation by Richard Wagner into a series of four operas, Der Ring des Nibelungen. The character of Siegfried and much of the plot of the medieval German epic originate from earlier Norse sagas, where the character is known as Sigurd.
Sigurd was requested to slay Fafnir by Regin, his foster parent and guardian. Regin was the brother of Otr and Fafnir and son of Hreidmar. Otr was killed by the Aesir after they mistook him for an otter, and as compensation had his body stuffed with gold. Fafnir then killed Hreidmar and stole the "Otr's Gold," and to better guard it, turned himself into a dragon.
Sigurd met Odin on his way, who advised the hero to dig trenches to drain the dragon's blood, which apparently gave him invulnerability. He killed Fafnir when the dragon walked over the trench, and bathed himself in the blood. However, a leaf had fallen and stuck itself on his shoulder. When he drank some of the blood, he understood the language of the birds and learned that Regin was plotting his death. As retaliation, he killed Regin first. Despite gaining the gold, however, it brought him no happiness.
Siegfried appears as a demon of the Famed race, which can be obtained through fusion accidents once the appropiate level is obtained. There is also a set of armor sold in a special shop of Ginza based on his armor, called the Nibelungen, divided in Helm, Legwear and Chest, which allows Flynn to reflect Physical attacks and null Death, at the cost of a weakness to Gun attacks.
Devil Summoner: Soul Hackers
"The name of the hero in the epic poem of the Nibelungenlied. The dragon Fafnir's blood made him invincible, but a single leaf on his back resulted in a weak spot."
—Soul Hackers 3DS compendium
Siegfried can be obtained if Zeed is level 50 or higher and fused with Lugh.
Devil Summoner: Raidou Kuzunoha vs. the Soulless Army
"The German name of the hero in the epic poem of the Nibelungenlied. The blood of the dragon Fafnir made him invincible, but a single leaf on his back resulted in a weak spot."
—Devil Summoner: Raidou Kuzunoha vs. the Soulless Army devil chart
"The German name of the hero in the epic poem of the Nibelungenlied. The dragon Fafnir's blood made him invincible, but a single leaf on his back resulted in a weak spot."
—Persona 3 compendium
Aside from returning as a Max Strength Social Link Persona, Siegfried can be fused with a Nihil Weapon to create the Balmung. He can also perform the Scarlet Havoc Fusion Spell with Ares.
Siegfried returns as the ultimate Persona of Strength Arcana. If one takes the newly added female perspective, maxing out the Koromaru Social Link will allow the fusion of Siegfried.
"The German name of the hero in the epic poem, "The Nibelungenlied." The dragon Fafnir's blood made him invincible, but a single leaf on his back resulted in a weak spot."
—Persona 5 background
Siegfried is the eighth Persona of the Strength Arcana and can only be obtained by fusion in the Velvet Room. He is one of five Personas to learn Auto-Mataru and the first of two Personas to learn Vorpal Blade. When itemized through Electric Chair execution, Siegfried yields a Vorpal Blade skill card.
Persona 5 Royal
Siegfried defends the corridor to the treasure chamber of Maruki's Palace. He is the last obstacle in its exploration, as well as the last mid-boss in the game.
DemiKids Light & Dark
"A legendary hero in shining armor who is armed with a sword called Gram."
Known as Zypher, he can perform the combo Razor Dance with Karfu and has the power Magnify.
Shin Megami Tensei: Liberation Dx2
"The name of the hero in the German epic poem Nibelungenlied. He became invincible but for a spot on his back when he was bathed in the blood of a legendary dragon he slew. In exchange for his sister's hand in marriage, the king of Burgundy asked Siegfried to help him trick Brunnhilde, the queen of Iceland, to marry him. Though successful, when Brunnhilde found out about the deception years later, she killed Siegfried out of spite."