Berserkers were Norse warriors who are reported in the Old Norse literature to have fought in a nearly uncontrollable, trance-like fury, a characteristic which later gave rise to the English word berserk. Berserkers are attested in numerous Old Norse sources. Most historians believe that berserkers worked themselves into a rage before battle, but some think that they might have consumed drugged foods. There are a few who suggest that they were just raging madmen.
The Úlfhéðnar, mentioned in the Vatnsdæla saga, Haraldskvæði, and the Völsunga saga, were said to wear the pelt of a wolf when they entered battle. Úlfhéðnar are sometimes described as Odin's special warriors, with the pelt from a wolf and a spear as distinguishing features.
"Fierce warriors of Norse origin who fight heedlessly and without fear. Their name comes from their bearskin coats that they wear when fighting. They are also warriors of Odin."
—Devil Survivor 2 compendium
Berserker is a unique demon that doesn't require a particular fusion combination to create. Instead, the protagonist has to achieve a Fate level of 3 with Keita Wakui before it is available. Once this has been achieved, it is unlocked for fusion on all subsequent runs as long as the player imports their save.
Devil Survivor 2: The Animation
As a nod towards Berserker's in-game ties with Keita's Fate Ranks, Berserker is Keita's signature demon, boasting incredible physical prowess that allows it to cut through swaths of demons. After Keita's death at the hands of Botis, Hinako obtains and uses Berserker in his memory, and its great physical strength lets the team severely damage Megrez's shell. It is also used to fight against Benetnasch, where it joined in with other demons to attack the full body. However, Berserker later gets forcibly sent back to its cell phone along with the other demons, thanks to Benetnasch's demon-revoking properties.
Shin Megami Tensei: Liberation Dx2
"Fierce Norse warriors who fight heedlessly and without fear. In battle their blood boils, atacking friend and foe with blind ferocity. Their name comes from the bearskin coats they adorn and are warriors of Odin."