Berserker is
Guan Yu,
the Chinese War God. Definitively the strongest and the most legendary of servants in this war, if his Original Master managed to control him, the war was defintively his. However he made a mistake. By directly aiming to summon him during his absolute peak, his Master summoned the version that represented his greatest feat: The Crossing of Five Paths and the Slaying of Six Generals. During this event, Guan Yu left his current master, Cao Cao, to return to his original master and brother, Liu Bei. And So, Guan Yu, with his memories jumbled, try to relive the journey, with Cao Cao being replaced by his current master and his original master being the Grail.
Caught by surprise, the Chinese Master isn't able to stop a rampaging Guan Yu, who also kidnapps a group of prostitute and homeless man who were kidnapped originally by the triad, thinking they are Liu Bei's entourage.
Guan Yu starts relatively strong and as more and more servants die in the war, the stronger he becomes. Without a Servant, the Chinese Master has to use other means to try to capture the strong general. He eventually teams up with the Team Caster. At some point he gets near to capturing the prostitute whoever Assassin who was raiding Caster's base, uses his Noble Phantasm to kill him.
Berserker actually survives until the end and is the one responsible for slaying the Final Boss after the reaches his strongest form. Guan Yu then fades away into the Grail with his substitute master with him