Direct and pretty much unmatched in melee combat, Berzerker is generally viewed as the biggest threat in the fifth grail war. In his past life he was Hercules. Illya doesn't bother to hide this fact as she's thoroughly confident that even such knowledge won't help anyone beat him. The berzerker class is said to be the most powerful, but also the most dangerous as they lack sanity and often turn on their masters. With Illya's skill and strength in magic, she's able to control him easily. With the strongest spirit combined with the strongest class; the the Einzbern family believed victory in the grail war would finally be theirs.
Berzerker never speaks. He behaves like a killing machine when fighting, but otherwise simply watches over Illya. It's not clear if this is a consequence of being the berzerker class, or how he was summoned by the Einzbern family. Berzerker does occasionally show signs of sentience: not just obedience It's rare though, and for the most part he just tears into whatever is in front of him on command.
Mostly Berzerker doesn't seem to need them. His raw power and speed are enough to overwhelm nearly any opponent. Even if killed, the job isn't done since his noble phantasm gives him twelve lives! With only 7 servants that are of much less power than him, it's easy to see why Berzerker is considered to be the toughest opponent.