Mystras Leoxses (ミストラス・レオクセス, Misutorasu.Reokusesu) was the older son of Darius Leoxses. He was a candidate for the next Knight-King of Sasan. He activated his Household Vessel of the Djinn Baal during a fight with Hinahoho, and became one of Sinbad's Household Members until his death during the battle of First Kingdom of Sindria. He was replaced by his younger brother Spartos as one of Sinbad's Eight Generals following his death.
He has bangs that go down the left side of his face. He wears a turban around his head, which has a feather sticking up from it.
Mystras is very curious and often sneaks out to hear stories about the outside world. He wants to leave his conservative home nation and travel the world, something his father and the knights don't approve.
Mystras possessed a tantamount of bravery and conviction, he made great effort from his training to be stronger. He was willing to give up everything just to see the outside world and challenged fighting every knight up to high-ranking until to his father, the Knight King, even the consequence of death if he loses.
Mystras shares some similarities with Sinbad, both have fun-loving and childish personalities, both are brave and convicted with their goals, both have perverted side and are fond of women.
As the oldest son of Darius, he was once in line to be the next Knight-King of Sasan until he fought his father for freedom to leave the Sasan Knights.
After failing to enter the Sasan Knights and being chased out by their reinforcements, Sinbad, Ja'far and Hinahoho retreat to the trade facility, where Mystras appears hiding behind a stack of barrels. He discreetly approaches Sinbad from behind and asks to sit with him. He states that he wants to talk with them about the outside world because he can't ask anyone but travelers from outside, as the people of Sasan are forbidden to visit the outside world by their doctrine. Mystras also added that especially for him, as a Knight apprentice, the precepts are very strict.
As they learned that he's a Knight, Sinbad and the others eagerly offer to help him with anything he requests, knowing that if they can build a relationship with him, they will gain a foothold into the Sasan Knights. Soon after, a female employee snatches Mystras up. She reminds him, multiple times already, that the trade facility is for foreigners. She says normal citizens, especially Knights, aren't allowed here. She tells Mystras to think of his position, not only just as a Knight but as the son of the Knight-King, which surprises the group. She introduces him to Sinbad and the others. Mystras also states that he is a candidate for the next Knight-King. Then, she tells Mystras to leave which he responds that he worked hard sneaking in and isn't going back until he gets to hear about the outside world. Sinbad intervenes and uses his charm to ask her to overlook this, to which she agrees.
Sinbad introduces himself to Mystras and hands him a book he wrote about his travels. He is amazed that someone who is the same age as him, wrote this. Mystras remarks that in a confined country like Sasan, there aren't any people like him. Sinbad responds by asking Mystras , if he dislikes Sasan. He replies that "I don’t know.. if it’s dislike.. in this country there's no freedom to decide your own way of living, and everyone's so serious and stuffy that it's boring. if I have to live stifled in this confined country, I'd rather die breathing freely in the wide world". Afterwards, Sinbad unsheathes his sword and activates his Metal Vessel which draws lightning down from the sky onto his sword. Then, he tells Mystras that it’s not just a story in a book and begins personally narrating the tale of the Adventures of Sinbad, amazing him and everyone inside of the facility.
Later that night, Mystras on his way back home, recalls his experiences with Sinbad and expresses a stronger desire to leave Sasan, and traveling the outside world with Sinbad. Then, his father, Darius Leoxses appears before him and tells him that he is too unmindful of his status as the son of the Knight-King. He states that he knew about his son's interest of the outside world for some time, only to remind him that Knights of Sasan are not allowed to leave, due to how they follow their doctrine unconditionally. He mentions his behavior has caused more than a few within the Knights who are displeased with him. He also asks Mystras, as an elder brother, if he intends to expose a shameful image to his brother, Spartos.
Mystras is surprised to see his brother and asks him what is he doing here. Spartos replies that he wanted to practice spear handling with his older brother he looks up to and asked to be let into the Knights' lodging house, then apologizes for troubling him. Darius asks Mystras again, if he's going to continue his foolish behavior in front of his brother, who adores him. Mystras remarks that his father only brought his little brother to make insinuations. He states he can't take it anymore and couldn't care less about the Knights or the Order. He can't stand this country, being secluded in the mountains and knowing nothing of the outside world, and spending his life with boring people. Mystras claims that he won't enter the Sasan Knights. Then Darius hits Mystras across the face, he looks back up to his father with strong resentment.
At the entrance of the Sasan Knights, the guards tells Sinbad, Ja'far and Hinahoho, they can't see Mystras. The guards mentions that they have no obligations to speak to outsiders like them but since the situation is urgent. They reveals that Mystras has officially declared his withdrawal from the Knights and for the Sasan Knights, once they're baptized as Knights, they must devoted their entire life to Sasan. No matter what the reason is, it's unforgivable to break that vow. For a Knight to resign is equivalent to a death sentence. The guards mention there is only one way a Knight can resign and walk away with his life.
Then, Mystras is shown spear fighting against a senior Knight in the halls of the throne room. Mystras dodges then intercepts the attacks which knocks his opponent's spear out of his hands, and points his spear at him. Mystras is declared the winner. The Knights in the room are stunned and comment on Mystras' ability and how it could rival the Chief Knights. They thought he will never be more than an apprentice because he always neglected his training and start to recognize him as the son of the Knight-King. They also reveal that Mystras has made it this far, all that's left is to challenge the Knight-King. If he defeats Darius then his position is relinquished to him, which will make Mystras the strongest person in Sasan and he will earn the free will to leave the Order. The Knights mention there hasn't been anyone who completed this ceremony yet. They acknowledge his serious intentions to leave the Knights. Afterwards, Mystras faces his father and recalls what the Knights talked about if he wins. Sinbad and the other makes their way through the crowd and see Mystras. He declares a challenge against Darius and states if he wins, he will be free.
The match between Mystras and Darius is announced. Ja'far tells Sinbad that whomever is defeated in the match will lose their head, according to the gatekeepers. The referee states that a fair match will be dictated by doctrine. The match begins, Mystras leaps to Darius and starts viciously attacking. He tells his father that in order to leave Sasan, he knew that he had to fight him one day and trained extensively. In order to defeat him, he had his father drilled his spearmanship into him since he was small child and studied his habits, tactics and counter moves. Mystras finally landed an attack on the Knight-King. Darius states that his son do have some skills that he can brag about however he reveals that his attacks will be send straight back to him and recites Alloces' name, which activates his Metal Vessel. Then, a barrier appears in front of him and Mystras is sent flying back by his own attacks. Darius praises his son for training himself well and how he manages to land a blow to the Knight-King but reminds him that no matter how well planned he is, it's futile to go up against "the power from the Gods".
The Knights believes that no one but the Knight-King, who conquered a "legendary tower" that appear at times when the world is full of unusual phenomena, is capable of wielding "the power of the Gods". Mystras gets back up and starts attacking again, knowing that the barrier are repelling all of his attacks. In a last attempt to win, he throws his spear towards Darius but was repelled however the spear was only a decoy and Mystras attempts to hit the Knight-King from the side that seems unprotected by the barrier. However, a 360 degree barrier appears around Darius and Mystras is sent flying back by his attack again, which surprises Sinbad and everyone in the room. Mystras is unable to continue the match, the Knight-King walks up to him. He tells him that even as an apprentice and negligent Knight, he will at least make a good offering to the Gods. Darius points his spear above Mystras and praised his brave efforts to leave the Sasan Knights, just when he was about to kill him. Sinbad intervenes and requests to stop the match. Sinbad tells Darius that he can't cheat in a sacred match, which offends the Knights, questioning how was their Knight-King cheating. Darius responds that this is a sacred match dictated by doctrine, there's no place for the trifling comments of outsiders here.
Sinbad questions their doctrine and says he's talking big for a guy that won with a Djinn's power. He further explains that, "You called it the power of the Gods because you got it for conquering some legendary tower? It's often been called that. It really is a special power but it's not a power that's just your.." Then, Sinbad unsheathes his sword and activates his Metal Vessel, which draws lightning from the sky onto his sword. He revealed that this is a power of a Djinn given to a Dungeon Capturer, all of them have it. He states It's not fair to use that kind of power to win against someone who's not a Dungeon Capturer and can't agree with it. Darius asks who he is. Sinbad properly introduces himself as a merchant traveling the seas of the world and like him, a Dungeon Capturer. He challenges the Knight-King to a match that if he win, he can deal with him as he wants. Darius asks him, what's he trying to accomplish. Sinbad replies that, If he wins, he will take Mystras and have Sasan trade with his company.
After Sinbad's overwhelming Victory for the Allegiance of his father Darius Leoxses, he pursued his will to leave Sasan. While heart warmed by both his father's apology for chaining him in the country, and hearing from him that he too wanted to see the outside world once as a child but not being able to say so, he let him go with sinbad as a representative of sasan, and also for him to experience the world in his place. After rejecting the apology stating that his father was an "immovable wall" that made him maintain his belief, he thanked him instead and departed with Sinbad.
Even when it was time for him to leave, his reluctant brother clung into him refusing to let him go stating that he admired his brother's resolve to go outside the city and respected him from the bottom of his heart, and if he was to leave he would be lonely. After hearing this, Mystras made a knight's promise to him that they would someday be reunited, and when Spartos becomes a Knight he would take on the same mission he had and would travel the world together.
He took on a more passive role while he acts as a representative of Sasan in the Sindria Trading Company.
Mystras was confirmed as the among sacrificed during the Battle of Sindria in its founding years. He was subsequently replaced by brother Spartos as one of the Eight Generals of Sindria. 
When he was a knight, he managed to defeat everyone with the exception of the Knight-King, in the Sasan Knights during the ceremony. During his match with the Knight-King, Mystras reveals that he learned his father's spearmanship and manages to land a blow to the Knight-King. He was defeated, however only due to the fact that Darius used his Djinn's power.
- Bararaq Harba (Flash Lance): Mystras' Household Vessel. It takes the form of his spear. When used, it sends a huge spear made of lightning at the opponent.
Mystras was bequeathed by his father to become the next successor as Knight-King. However, Mystras has no desires to become a Knight and wishes only to explores the outside world. He continues to neglect his training and sneaks out of the Knights to hear stories of the outside world, which puts a strain in their relationship.
Spartos, his younger brother, looks up to him as a role model.
They both seem to have fun together, especially when talking about deviant things. Mystras looks up to Sinbad.
- His name might derive from the Greek town of Mystras (Μυστράς), which is a part of the municipality of Sparti (Modern Sparta).