Darius Leoxses is the former Knight-King of the Sasan Kingdom, a religious and isolated nation that is part of the Seven Seas Alliance led by Sinbad. As a Dungeon Capturer, he is the owner of the 52nd Dungeons djinn, Alloces, making him a King Vessel. In terms of appearance, Darius is a middle-aged man with a serious expression, often described as having cunning eyes, a small beard, and dark red hair. He typically wears ornate clothing with metallic armor on his arms and legs and carries a long metal spear, which serves as his Metal Vessel. He also wears a light-colored turban on his head.

Before meeting Sinbad, Darius was an exceptionally strict and devoted ruler who followed the doctrines and religion of Sasan without question. He was a devoted isolationist, enforcing the country's hundred-year policy of seclusion due to the belief that the outside world was impure. This strictness profoundly affected his relationship with his sons, particularly his eldest, Mystras Leoxses, whom he forbade from leaving the kingdom and raised with excessive severity.

Darius underwent a significant transformation following his encounter with Sinbad. When the Sindria Chamber of Commerce arrived in Sasan, Darius clashed with Sinbad after being forced to use his djinns power to subdue his son Mystras, who sought to leave the country. After witnessing the overwhelming power of Sinbad, Darius was humbled and realized the vulnerability of his kingdom should it face a real threat. This experience made him more modest and open to change. He not only permitted Mystras to travel with Sinbad but also lifted Sasan's isolation policy and joined the Seven Seas Alliance. As a result, he grew into a more caring father and a ruler capable of change while remaining deeply protective of his kingdom.

His primary motivation shifted from preserving the purity of his kingdom to protecting it through cooperation with the outside world. As a member of the Seven Seas Alliance, he became a trusted ally of Sinbad. During the Magnostadt Arc, Darius fought alongside the other King Vessels against the Medium, performing Full Djinn Equip and Extreme Magic. He also participated in the Kou Empire Arc, coming to the aid of Hakuryuu against Kou Empire forces. In later events, Darius chose to abdicate his throne, passing the rule of Sasan to his younger son, Spartos, and joined the newly formed International Alliance as a member of its Board of Directors.

The key relationships in his life are with his sons. He has a complicated history with Mystras, having been both jealous of his son's eagerness to see the world and ultimately apologetic for his own reluctance to leave Sasan. His other son, Spartos, initially lived in Sindria before eventually returning to Sasan to succeed his father as the new Knight-King. His relationship with Sinbad is also central, as Sinbad served as the catalyst for his personal growth and the opening of his country.

In terms of notable abilities, Darius is a skilled spearman, using his spear as his primary weapon and Metal Vessel. His djinn, Alloces, grants him potent defensive capabilities. When he performs a Djinn Equip, his appearance becomes much younger, shedding his beard and wrinkles as he dons a full set of dark armor and often rides a dark, armored horse. In this form, his spear transforms into a thinner, curved weapon resembling a guandao, with the specific ability to create powerful barriers that can deflect enemy attacks.