Hagen is a Burgundian warrior in tales about the Burgundian kingdom of Worms. Hagen is often identified as a brother or half-brother of King Gunther.
In the Nibelungenlied, he is called Hagen of Tronje (perhaps Drongen in Ghent). In German tradition, Hagen is especially grim, implacable and violent, and in two accounts one-eyed. According to the Thidreks saga, Hagen was not fully human, being fathered by an elf with the king's wife. In these accounts, it is Hagen who kills the hero Siegfried during a hunt, wounding him on the only part of his body which was not invulnerable. This version of the character appears in Wagner's Der Ring des Nibelungen (in this version, Hagen drowns trying to steal the Rhinegold). In Norse tradition, Hagen's counterpart Högni is less extreme and the actual slayer of Sigurd (the Norse counterpart to Siegfried) is Gutthorm, a younger brother of Gunnar and Högni. But Gutthorm does so when egged on by his elder brothers.
In German accounts, Gunther and Hagen are the last survivors of the fall of the Nibelungs. Hagen refuses to reveal the hiding place of the Nibelung treasure as long as his King Gunther lives. When Gunther is slain, the mortally-wounded Hagen continues his refusal with a sure knowledge that Gunther cannot now weaken and betray the secret. In Norse accounts, however, it is Gunnar who refuses to tell the secret as long as Högni lives, and so brings about Högni's death.
In Atlamál, Hniflung a son of Hagen/Högni avenges his father's death and the deaths of his kin together with his aunt Guðrún. This work also states that Hogni had a wife named Kostbera and two other sons: Solar and Snævar. The Drap Niflunga mentions a fourth son named Gjuki (named after Hogni's father).
"The warrior who killed Siegfried in the Nibelungenlied.
He was a loyal vassal of Gunther, king of Burgundy. He killed Siegfried by throwing a spear at Siegfried's only weak spot while he was drinking from a brook. Afterwards, he and his king were both beheaded by Siegfried's wife Kriemhild."
—Shin Megami Tensei IV compendium
Hagen is a member of the Famed Race, and as such can only be obtained through a fusion accident. Fusing a KO'd Mad Gasser with a KO'd Wild Hunt will result in a high chance of a fusion accident that can potentially summon Hagen. Hagen can teach Flynn the Berserker God skill through his Demon Whisper, along with any that were inherited from the parent demons.
"The warrior who killed Siegfried in the Nibelunglied. He was a loyal vassal of Gunther, king of Burgundy. He killed Siegfried by throwing a spear at Siegfried's only weak spot while he was drinking from a brook. Afterwards, he and his king were both beheaded by Siegfried's wife Kriemhild."
—Soul Hackers 3DS compendium
Hagen can be obtained if Zeed is level 50 or higher and fused with a Fafnir during a new moon.
"From the Nibelunglied, the warrior who killed Siegfried. He was a loyal vassal of Gunther, king of Burgundy. He killed Siegfried by throwing a spear at Siegfried's only weak spot while he was drinking from a brook. Afterwards, he and his king were both beheaded by Siegfried's wife Krimhild."
—Devil Survivor 2 compendium
As well as the other members of the Hero Race, Hagen is only obtainable with a unique fusion between Berserker and Ictinike.
Hagen appears as a demon under the ownership of by Ronaldo Kuriki. Like in the original game, Hagen seems to favor physical combat over magic, using his sword to slice through enemies and defend itself. He first appears as the demon that finished off Merak before it could try to attack Hibiki one last time. He was then used in other scuffles such as against Airi Ban's Cait Sith, the battle against Phecda (in which he was easily destroyed), and the battle against one of the Megrez buds. Hagen contributed significantly against Megrez, as he destroyed one of its cores simultaneously alongside Hibiki's Byakko and Yamato Hotsuin's Cerberus.