"He was a childhood friend of Rena, but a certain incident has left a deep scar within him. Traveling in search of strength, he became a vagrant swordsman. Cool-headed and unable to trust others, his skills as a swordsman are considered to be the greatest."
—STAR OCEAN: Second Departure Official Website.
Dias was born and raised in Arlia, and is Rena's childhood friend. He thinks of Rena as his "other little sister" since she would often play with Dias's younger sister, Cecille.
Two years prior to the events of the storyline, Dias and his family were attacked by bandits. His parents and sister died, and Dias himself was badly injured, but he managed to survive. After that incident, however, his life changed: he came to believe he was somehow responsible for the death of his family, for not being able to protect them. Dias then left Arlia and travelled all over Expel, becoming a renowned swordsman.
"Call me what you will, but I don't feel like babysitting a bunch of people who will be nothing but a burden to me."
Dias is met for the first time in Marze, when the children of Marze are held captive in the Forest of Symbols and a plan is being devised to rescue them. As it turned out, Dias was hired to attempt to rescue the children. If Rena is the chosen protagonist, Dias will join temporarily to help Rena carry out the rescue mission. After the events that unfold, Dias says to Claude that he anticipates the day their swords will cross, and he heads off again.
Dias later reappears in Lacuer during the Lacuer Armory Contest, in which he is a participant. It is then that Dias and Claude finally face off against each other. Not surprisingly, Claude loses the fight against Dias. Nevertheless, Dias acknowledges his strength, and says he looks forward to seeing him again.
"I'm telling you, he's first-class swordsman. He'd be a huge asset to us out there!"
—Rena Lanford pleading her case to Claude Kenny and Celine Jules.
Near the end of the first half of the game, at the Lacuer Front Line Base, Dias runs into Claude and Rena once again. If Claude is the protagonist, he and Dias will ensue in a short conversation, after which Dias asks Claude to protect Rena for him, and then he leaves. If Rena is the chosen protagonist, however, the player can opt to make Dias a permanent addition to the party.
Star Ocean: Blue Sphere
Little is known about where Dias went during the two years between the games. It is likely he reverted to his old ways of wandering the world alone, looking to better himself and his sword.
Precis manages to find him and ask him to join the cause. Dias then volunteers to join, so that he may protect them himself. He mostly joins out of duty to Rena, wanting to continue to protect her.
"Going on an adventure doesn't really change me or anything. I keep pursuing greater and greater levels of strength, and in the end, I've accomplished nothing."
An expressionless swordsman, Dias' eyes sparkle with a cold light. He is cold, callus, and arrogantly proud. The blue-haired warrior enjoys practicing his own unique swordfighting style that he developed himself, meshing the Edarl sword style with the martial arts of Karate. Disliking bandits and robbers with a hatred born of the murders of his family, Dias enjoys simple pleasures such as food and sleep.
"He may not like dealing with others, but he causes chaos everywhere he goes regardless."
Dias uses a fighting style very similar to iaido, the Japanese sword-based martial art associated with unsheathing and re-sheathing one's sword for quick-draw-based strikes. He is often considered the most powerful character in the game, tied with Claude, Ashton and Welch. All his stats are very high (although his CON is much lower than Claude's), while also being a notably mobile character, having the second highest faster movement speed (after Chisato).
Most of his Special Arts are fast and deadly, usually with a low MP consumption, and reach their Top Proficiency fast, meaning they achieve full potential in short time; however, Dias tends to fall a bit behind in damage output compared to other fighters as many of his Arts lack high multi-hit properties. His normal ground attack has one of the highest ranges, although a slow start-up. Among his notable Arts are Air Slash, which is much faster than Claude's, being the staple of his gameplay until the end of the game; and Deadly Illusion, which is the fastest Special Art in the game.
In Second Story, the accessories Meteor Ring and Slayer Ring had no effect on Dias. This has been fixed in Second Evolution.