Before Kingdom Hearts Birth by Sleep
Having been instructed by his master to transfer his heart from one vessel to the next until the fated "Dark Master" appeared to reenact the Keyblade War, Luxu's final role was to become an ally of this individual, Master Xehanort, and aid in his plans in order to continue to observe the future. Luxu eventually finds Braig as his final vessel for his heart.
Kingdom Hearts Birth by Sleep
Braig had long been one of the guards and apprentices to the king of Radiant Garden: Ansem the Wise. At one point, he meets Master Xehanort, who tells him of his plans to create the χ-blade. Braig agrees to become one of Xehanort's thirteen seekers of darkness in exchange for the power to wield a Keyblade. As part of their bargain, Braig must pretend to take Xehanort captive to force Terra to embrace his darkness so Xehanort could transfer his heart Terra's body to gain a youthful vessel.
Braig finds Terra after he separates from Aqua and Ventus, and he leads the skeptical Terra toward Radiant Garden's Purification Factory, where he shows him Master Xehanort chained to a large pole, much to Terra's horror. Braig then engages Terra in a duel, intent on eliminating him and taking his Keyblade. Terra is reluctant to fight back out of fear of succumbing to the darkness and endangering Master Xehanort, until the latter screams for him to use the Keyblade and avoid shaming Master Eraqus and Terra's fellow pupils. Terra fights back and eventually defeats Braig by using the power of darkness, in the process scarring Braig's face and permanently blinding his right eye. After Terra leaves, a bandaged Braig furiously attempts to gun down Xehanort for setting him up as "collateral damage", but Master Xehanort threatens him into backing down and continuing with their plan. Some time later, Xehanort infuses a piece of his heart into Braig's.
Braig later appears in the Keyblade Graveyard, his body having adopted Master Xehanort's yellow eyes and pointed ears; his hair has also greyed somewhat, and he now has a patch over his eye. He appears in front of Aqua and Ventus, who taunts them and attempts to turn them against Terra by bringing up Eraqus's demise. After mocking the angry glare Ventus gives him, Braig is challenged by Aqua and defeated. Since he had only needed to buy time for Vanitas to arrive on the scene, Braig promptly flees. Aqua gives chase, but she turns back, not wanting to leave Ventus alone in his vulnerable state, while Braig returns to Radiant Garden.
Braig later discovers Terra-Xehanort and Aqua had fought in Radiant Garden, leading Dilan and Ansem to an amnesiac young man who resembles Terra, but with silver hair. When the young man is asked for his name, he answers "Xehanort". Braig smiles triumphantly to himself, satisfied Xehanort succeeded in taking over Terra's body. He asks Dilan to gather Aqua's discarded Keyblade Armor and Keyblade as he carries Terra-Xehanort to the castle on Ansem's order.
Braig later approaches the new Xehanort, now an apprentice under Ansem, and is dismayed to find that the new Xehanort does not recognize him, and Braig's attempts to help Xehanort remember serve only to annoy him. At one point, Braig questions if this new Xehanort is actually Terra, but Xehanort still does not respond, much to Braig's relief. Nevertheless, Braig promises to look out for Xehanort.
Between Kingdom Hearts Birth by Sleep and Kingdom Hearts 358/2 Days
After Ansem began performing tests on Xehanort's heart, Braig convinces the other apprentices to aid him and Xehanort in conducting inhumane experiments on the human heart and the darkness within it, in the attempt to remove one's sense of self, which would allow them to create vessels for Master Xehanort's heart. These experiments led to the mass release of Heartless into the Realm of Light. Among the subjects was a girl dubbed "Subject X" with no memory of her past aside from the phrase "May your heart by your guiding key" and that she needed to meet five certain people again. Braig freed her for an unknown reason. 
Soon after, Braig, Xehanort, Dilan, Aeleus, and Even banished Ansem the Wise to the Realm of Darkness, telling Ienzo that Ansem had went mad and abandoned them. They continue their experiments in the meantime, until Even and Ienzo are overcome by darkness in Ansem's computer room. Braig demands an explanation from Xehanort, who summons his Keyblade and stabs Braig in the chest, removing his heart. Not long afterwards, Xehanort, taking the name "Ansem", gave his own heart over to darkness, becoming a Heartless and Nobody. Although his heart was removed and became a Heartless, Braig's strong will enabled his body to continue existing as the Nobody Xigbar. Under Xemnas, Xehanort's Nobody, and Xigbar, the Nobodies of the six apprentices formed Organization XIII.
Kingdom Hearts Re:coded
Following the destruction of Organization XIII and their Heartless counterparts, Braig, Dilan, Even, Aeleus, Ienzo, Isa, and Lea are restored within Ansem's Study. Braig is awakened by a time-traveling Young Xehanort, and realizes that he must become Xigbar again to continue Master Xehanort's plan. The two discuss the plan for a short period, then agree to bring Isa with them, as Saïx.
Kingdom Hearts 3D: Dream Drop Distance
Later, Xigbar reveals Braig's pact with Xehanort to Sora, as well as the fact that he himself is "already half-Xehanort."
Braig wears a guard's uniform with a dark slate grey jacket, silver highlights, and two rows of gold buttons on the front. He wears a black belt and white gloves adorned with a silver, curled heart symbol. His jacket bears some resemblance to Xigbar's black coat, sporting the same pointed shoulders. He also wears black, knee-high boots and a tattered, red scarf. In terms of facial features, Braig sports brown eyes, though they turn gold upon his acceptance of a piece of Master Xehanort's heart, upturned eyebrows, and dark, shoulder-length hair that he wears slicked back. After his fight against Terra, he loses his right eye and receives a jagged scar over his left cheek, making him and Master Eraqus the only two characters that are seen receiving scars during the story. After he takes the bandages off, Braig wears an eyepatch to cover his lost eye. Braig's ears also gain noticeable points after he accepts a piece of Master Xehanort's heart.
Braig wields two Arrowguns shaped like crossbows. They are colored in shades of silver and a faded, metallic purple and adorned with the same curled heart symbol as his gloves.
Since Braig is the original persona of Xigbar, he possesses most of Xigbar's personality. He is shown to have a laid-back, cocky attitude, and he often does not take things seriously. Braig often refers to Master Xehanort as an "old coot" and, like Xigbar, punctuates his sentences with, "As if." Braig appears be cruel and sadistic at times, as seen when he nonchalantly approaches Aqua and Ventus and asks which one of them wants to be destroyed first. He is a skilled manipulator, easily goading Terra into fighting him and successfully stalling for time in his battle with Aqua.
Braig has a deep fascination with the Keyblade and his desire to possess one of his own is what led him to join Master Xehanort in the first place. Ultimately, he allowed Master Xehanort to begin transforming him into one of his twelve vessels just to gain the ability to wield a Keyblade, even at the eventual cost of his individuality. Though normally loyal to Xehanort, Braig attempted to attack the old man after his fight with Terra, angry that he had been hurt despite Xehanort's guarantee to the contrary.
The end of Kingdom Hearts III, however, reveals that Braig and Xigbar's personality is an entire ruse and he is a vessel of the heart of Luxu, who put on the facade in order to get close to Master Xehanort.
Like Xigbar, Braig wields twin Arrowguns in battle, using them to fire a variety of quick shots. He uses a telescopic view in battle and is able to teleport, warp his bullets, and levitate upside-down via space distortion. As with his Nobody's, Braig's HUD display has a silhouette of his Arrowguns in each of the four cardinal directions. The shots Braig fires when he is sniping are violet like Xigbar's, but they seem to have explosive properties to them. He can also combine his two weapons into one in order to fire powerful shots in rapid succession. Braig is also very agile, able to make high leaps into the air, and quickly run around opponents while leaving after-images of himself.
In the Western and Final Mix releases of Kingdom Hearts Birth by Sleep, his targeting reticule was changed drastically, much like it was for his Nobody, Xigbar. Instead of the crosshair-shaped reticule used when fighting Terra, it simply showed three violet dots, vaguely showing Braig's aim.
Notes and references
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