Titan Fighter or Titan Mauler using a Large Greatsword would probably be best. His sword was, explicitly, too big for a normal person to wield. He'd probably also have variant multiclass Sorcerer to get Abyssal bloodline, primarily for the Str bonus (he draws extra strength from the curse mark). And I second shaman's idea of using Slayer to represent his "assassin" workings; remember, you don't need to be a Rogue in order to do stealthy stuff. He's also quite above-average compared to most other normal people. He's not exactly a genius, but not exactly a dullard, either. He's probably got 13 Int and both Combat Expertise and Power Attack so he can easily switch between a defensive stance and offensive stance. Even though the series name is "Berserk", he doesn't always emphasize the "berserk rage" we come to expect of a barbarian; Gatsu has always been about staying power, remaining in the fight long after others have exhausted themselves. That speaks to me more of Fighter than Barbarian. The extra Fighter feats also cover what he loses in taking the variant multiclass. So I'd say Titan Fighter w/ VMC Sorcerer, plus a few levels of Slayer is best, Str and Con super-high, Dex decent, dump Cha, but put skill points into Intimidate and also take Intimidating Prowess. And maybe a custom armor that lets him rage.
