Black coat

The black coat (黒いコート), also called the black cloak, is a magical form of clothing that allows the person that wears it to safely pass through the Corridors of Darkness; appearance-wise, it is simply a black long-sleeved, double-breasted, twin-tailed trenchcoat with a hood. It has a large silver zipper that fastens at the top and zips down to seal the coat. It has silver drawstrings for the hood decorated with a large silver bead hanging from the end and a silver chain ornament adorned with four large cylindrical silver beads that fastens to a loop on either side of the collarbone region.

It is most noticeable as the uniform for Organization XIII, but it is first used by Master Xehanort in Kingdom Hearts Birth by Sleep as an alternative to the Keyblade wielders' traditional armor. Several characters outside of the Organization wear this coat, including Riku, King Mickey, Ansem the Wise, Anti Black Coat, and a Moogle.

The Black Coat's main purpose is to resist the corruption inflicted upon one's heart when they pass through a Corridor of Darkness. It also seems to be able to change shape to fit its wearer, as seen when Xion shifts from her normal form to Sora's, and when Riku changes into his Ansem form.

Somehow, when Riku and King Mickey cast their coats off, the black gloves, pants, and boots worn under the coat disappear. Furthermore, any trace of the wearer's clothing under the coat disappears, such as Riku's collar, jeans, and sneakers in his second attire, or Mickey's trademark white gloves. All these hint that donning the coat transforms whatever clothing the coat's bearer is wearing under it into the black gloves, pants, and boots seen on all the coat's wearers. However, Roxas's sneakers are visible under his coat in several of his appearances.

Development
The black coats were originally designed for Roxas and Riku in the secret video Another side, Another story... simply as a way to disguise them while they were still being designed for Kingdom Hearts II. It was again used simply to hide Xemnas's face during his optional boss battle in Kingdom Hearts Final Mix and in Deep Dive, starting the trend of all Organization members wearing it. Nomura stated in an interview that his initial idea for "The Gathering" was to have Terra, Aqua and Ventus wear similar coats to the Organization XIII coats to hide their at-the-time undesigned faces, but felt that he could not use black coats once again, and so chose armor instead.

Story
The first known use of the black coat was by Master Xehanort, who wore it when he traveled from the Keyblade Graveyard in order to leave his comatose apprentice, Ventus, to expire at his boyhood home of the Destiny Islands.

Years later, at the same time Master Xehanort attempts to reforge the χ-blade, his time-traveling younger self arrives at the Land of Departure on his path to assembling Xehanort's true Organization XIII to replace the initial version that had failed in the future. He is found by and battles Ventus and his friends Terra and Aqua when each of them returns, individually, to that world, but after being defeated he simply vanishes on his way further into the future.

Even later, after Master Xehanort became Terra-Xehanort, and eventually the Nobody Xemnas, his half-remembered recollections of his past led him to adopt the black coat as the uniform of his Organization XIII. Each member wears their coat throughout their service. When Riku and King Mickey leave the Organization's former stronghold of Castle Oblivion, DiZ gifts them with their own black coats, so that they are able to safely pass through Corridors of Darkness and evade the Organization. The pair wear their cloaks through multiple clashes with the Organization, culminating in the capture of Roxas at Dark City, just after the former Organization member had deserted. Roxas is placed within DiZ's simulated Twilight Town as a normal human, making him the first Organization member to go without his coat. Mickey eventually discards his cloak when he rejoins Sora's group at the Battle of 1000 Heartless, while Riku continues to wear his until shortly after he rejoins his friends at The World That Never Was, when the explosion of the Kingdom Hearts Encoder returns him to his original form. When the group reaches Xemnas, the Superior replaces his black coat with his old Keyblade Armor, then with a modified version of the coat featuring white stripes.

Shortly after the Organization had been defeated, King Mickey digitalizes Jiminy's Journal in order to determine the source of a mysterious message that had appeared within it. In doing so, the Journal's new digital consciousness takes the form of Riku as he was after he left Castle Oblivion, wearing a black coat.

Meanwhile, several of the former members of the Organization revived as humans back at the Radiant Garden, and while most of them were wearing the lab coats or uniforms they had on when Terra-Xehanort transformed them into Nobodies, the human who had once been Axel, Lea, continued to wear his black coat as he left to seek Isa and assist Sora. At the same time, Young Xehanort arrives from the past with several of the members of the new, true Organization XIII composed of Xehanort's various incarnations. As they interfere with Sora and Riku's Mark of Mastery exam and maneuver Sora towards the gathering at Where Nothing Gathers, each of the members bar Ansem wear the coats symbolizing their service; Ansem finally dons his coat at the meeting itself. Even a solitary apparition of Riku's darkness who appears before Riku in Monstro wears the coat. In the final stages of their plot, the Organization places Sora under the influence of a Nightmare in order to prepare him as their thirteenth member. When Riku attempts to free Sora, the Nightmare takes the form of a shadowy figure in a violet-stained black coat, named Anti Black Coat.

Organization XIII
Among the most notable characters that sport the black coat are the members of Organization XIII. The Organization's coats are not uniform in appearance. For instance, Axel's coat has tight sleeves, while Xigbar and Demyx both wear coats with pointed shoulders. Additionally, Vexen's coat is longer than most other members, to the point that it covers his feet and its sleeves partially cover his hands. Roxas and Xion in Sora's form wear what appear to be black sneakers, while Larxene and Xion in her normal form wear heeled boots.