Berserker is a large man, towering over every other character in the story. Appearing in a technologically advanced suit of tan and red armor provided by the military, it has several distinct features, most notably several thick cables hanging from the upper back and a large, bucket-like helmet that completely encases his head, six horizontal slits aligned in two vertical columns serving as his means of seeing out. Beneath the armor, he wears a suit of deep green.
After losing his helmet, it is revealed that he is a fairly young looking man with spiky crimson hair swept backwards, his fringe normally hanging in front of one of his eyes.
Mori Nagayoshi was known for having a very violent temper and ruthless behaviour in combat, leading to his nickname.
After being summoned Berserker is initially restrained in a lab in Demon Archer's compound. He eventually is released and begins to rampage through the compound, cutting off Shinji's arm to remove the threat of Command Spells in the process and then carrying him while slaughtering guards.
Encountering Lancer and his master, the two battle, Lancer gaining the upper hand after splitting Berserker's helmet and piercing his head, seemingly killing him. However, his armour revives him, and activating his Noble Phantasm, he catches Lancer off guard. However, before their fight can reach a conclusion, Berserker is struck from behind by Sakura Saber.
Fighting both Lancer and Saber together, Saber confronts Berserker with her Lightless, Three-Stage Thrust, piercing clear through Berserker's heart. Unable to recover, Berserker fades away.
Berserker's Noble Phantasm is his large spear, Boneless Man, that he boasts can cut through any armor. He was given armor by the army that is capable of absorbing magical energy from the land, allowing him to act without need of worrying of draining his Master. It also provides regenerative properties, allowing him to revive from even fatal injuries unless he is destroyed all at once by something on the level of an Anti-Army Noble Phantasm. The armor will eventually kill the land with enough use, but the purpose of Berserker was to be an annihilation weapon to be dropped into enemy territory. He would also possess the Noble Phantasm 百段（ひゃくだん） (, ?), his favored horse, in the Lancer or Rider classes.