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Dias Flac is a powerful swordsman on the planet Expel in Star Ocean: The Second Story. He hails from the same village as Rena, Arlia. A few years prior to the game's events, Dias' entire family was brutally murdered by bandits, who nearly killed Dias and left him for dead. He has since held a deep hatred for bandits.
Early in the story, Dias is seen as something of a rival to Claude, having been garnering Rena's attention. Claude is at first threatened by him, but later learns he considers Rena more of a "little sister", and they become friends. Dias often meets up with the party to aid them in dangerous situations, or test his battling skills. He first joins them in the village of Mars, where he and Rena split off from Claude and Celine, both groups going into the forest to take out the bandits and save some kidnapped children. They meet following the defeat of the bandit leader, and it seems that Rena and Dias had already freed the children.
Dias later shows up at the Tournament in Lacour, where he shows his true might, making even Claude (who didn't like him at the time) to praise his immense skill and power. If you win all your battles in the tournament with Claude, you can even battle Dias yourself, to really compare strength.
Dias later meets the party at the front lines on the Lacour continent, where he joins in defending the continent from a wave of powerful creatures led by the mysterious and deadly Shin. He then parts ways with the party again, making Claude promise to take care of Rena and protect his "little sister".
Depending on which story you choose, you may or may not be able to get Dias as a character. If you play Claude's story, you won't be able to recruit him, but in Rena's story, he is recruitable. Early on he is a great help, being one of the more powerful characters in the game, but later into the game his strength is no longer superior to all the other characters, and his replacement is likely.