Darios is a prince of the Gristonne Kingdom and the son of Ozosid. He is the one who taught Rowan how to use a sword and taught Lianna the academic arts. The twins treat him like an older brother.
Darios arrives in Aytolis at the beginning of the game, and spars Rowan while Lianna watches. After defeating Rowan, Lianna is about to spar with Darius herself when the Gates of the Other World suddenly open, causing monsters to flood the palace. The three work together to fight off the monsters, and meet up with Queen Yelena to make their escape. However, while running through a cave, Yelena is separated from the rest by large boulders, and gives the Shield of Flames to Rowan and Lianna before urging them to go on without her.
After that, Darios continues serving as a mentor and ally for the twins as they find heroes of other worlds to power up the Shield of Flames.
During the the heroes' attack on a Gristonne fortress, Darios is trapped inside the castle and unable to escape from the enemies’ barrier, thus ended up being possessed by the dragon Velezark from behind before the heroes arrived and rescue Darios. After all heroic orb has been gathered into the Shield of Flame, Velezark, using the body of Darius began to initiate its plan to steal the shield from the heroes and revealing itself has been conspiring with Ozosid to acquire the Shield of Flames for their own evil purposes. Velezark permanently takes Darios’ body as its own and takes the Shield of Flames, and the twins pursue him.
When they catch up to him, they find that Ozosid and Velezark possessed Darios also have Yelena hostage. Ozosid and the possessed Darios prepare to sacrifice Yelena to revive Velezark’s full power, though the possessed Darius makes the proposal that if the twins surrender their weapons, he will spare her. When they do so, he turns to kill her anyways, but is stopped by Takumi, who predicted this outcome. The twins retrieve their weapons, and Rowan forces the possessed Darios back as Lianna rescues Yelena from the magic circle she was being held captive in.
With Yelena rescued, the twins attempt to get Darios back from Velezark’s possessions when Ozosid approaches him. The possessed Darius reassures his father before suddenly stabbing him in the stomach, saying that all he needed was royal blood. With Ozosid as the sacrifice in Yelena's place, Ozosid's body is absorbed by the magic circle to fuel Velezark's full power into Darius’ body. Thankfully, during the battle against his fellow heroes, Darios manage to fight back Velezark from the inside for the heroes to defeat him.
After the twins defeat him, Darios fully regains control over himself and pleads for them to kill him before Velezark manifest itself from his body. They refuse to, and believing there must another way to save Darius, but was too late, just as Velezark began to separate from Darios’ body, using the Guston prince’s life force to materialize its body. Although Darios is freed from the dragon's possession, it is short lived as Darios pushes the twins away from one of the dragon's attacks and is impaled by a sword. Using the last of his strength to return the Shield of Flames, the floor under him gives way and he crashes to the depths below as the twins scream his name.
He shares some similarities to Gunter, as both good warriors were initially revealed to have been possessed by an evil dragon on certain last chapters of their respective game series. Unlike Gunter, Darios is killed in the battle.
Darios also shares many similarities with Lyon; both are princes off their respective kingdoms who have close friendships with the twins of a neighboring country. Both princes are revealed to be possessed by the final bosses of their games and steal the protagonists' symbol of hope (Lyon destroys it, in his case). After their possessed selves are defeated for the last time, they both die apologizing for the trouble they've brought the twins.