Members get their number in the order in which they joined. The actions of Sora and Roxas have reduced the Organization's numbers to 7 before they invaded the Organization's HQ. Here's the order in which they joined. If you remove the X in their names it should spell the name they had before they became a nobody.
Each Members Ability
No. 1- Xemnas controls sorcerer nobodies and he uses Aerial Blades in battle to fight (similar to lightsabers). He also uses the power of nothingness to help him in battle. He's the nobody of Xehanort/Terra.
No. 2- Xigbar is also called The Freeshooter. Xigbar controls Sniper nobodies and uses his Gun Arrows in battle. He can combine his guns to make a sniper rifle or just leave them be. He manipulates space to teleport himself, his bullets, and turn upside down. He is the nobody of Braig who is a true ally of Xehanort.
No. 3- Xaldin is also known as The Six Lances of The Whirlwind. Xaldin controls the power of wind and his six lances to fight. He can also transform into a dragon attacking people with whirlwinds. He controls Dragoon nobodies. He is the nobody of Dilan.
No. 4- Vexen is known as Chilly Academic. Vexen can control the power of ice and he carries a very large shield into battle that blocks absolutly every frontal attack that anyone throws at him. Vexen can create anything he gathers data on. He created a replica of Riku using the data he had on him. He is the nobody of Even.
No. 5- Lexaeus is known as the Silent Hero. Lexaeus controls the element of earth. Lexaeus also uses a large tomahawk to fight his enemies. Lexaeus also tries to make Riku give in to the darkness but gets killed by him. He is the nobody of Aeleus.
No. 6- Zexion is known as the Cloaked Schemer. Zexion controls the power of illusion to gain the edge in battle over his enemies. Zexion can mimic other people's weapons, change himself to look like others or duplicate himself. His weapon is a lexicon. He is the nobody of Ienzo.
No. 7- Sai'X is known as the Luna Divider. Saix has an X shaped scar on his face and wields a claymore. Sai'X gets his power from the moon and he controls Berserker nobodies who have weapons like his. When he gets power form the moon he goes in a berserk state where he just attacks wildly with his claymore. If someone takes his weapon while he's going back to it they go berserk and attack. He was loyal to Xehanort. He is the nobody of Isa.
No. 8- Axel is known as the Flurry of Dancing Flames. He wields two chakrams and controls the element of fire. Axel controls the Assassin nobodies. Axel also has a strong friendship bond with No. 13, Roxas.Axel can create walls of flame or he can throw his weapons while they are on fire to damage the opponent even more. He is the nobody of Lea.
No. 9- Demyx is known as the Melodious Nocturne. Demyx uses the power of water and his control over Dancer nobodies to fight. Demyx's weapon of choice is a sitar which he uses to create duplicates of himself for his opponent to fight. Demyx fades away into darkness after he is defeated by Sora, Donald, and Goofy at Hollow Bastion.
No. 10- Luxord is known as the Gambler of Fate. Luxord uses his power over time and his command of Gambler nobodies to fight his opponents in battle. Luxord can also turn people into a card or a die and he can use his cards as weapons or for defense. Luxord fades into darkness after Sora defeats him in The World that Never Was.
No. 11- Marluxia is known as the Graceful Assassin. Marluxia uses his sycthe and his power over flowers to attack his opponents. Marluxia and Larxene tried to take over the organization at one point in Castle Oblivion. Marluxia and Larxene failed though when Sora managed to beat both of them and they died.
No. 12- Larxene is known as the Savage Nymph. She's quick and she uses the power of lightning in battle to fight her opponents. She seems weak with water, referencing her as a witch. She along side Marluxia tried to take over the organization in Castle Oblivion by using Sora and his power. She however fails when she is killed by Sora, Donald, and Goofy.
No. 13- Roxas is known as the Key of Destiny and he is the newest recruit of Organization XIII. In battle Roxas uses the Oblivion and Oathkeeper keyblades and he also uses the power of light in battle. Roxas being one of the stronger Nobodies he controls the Saumrai nobodies in battle. He's friends with Axel and "feels" like an outcast in the organization plus he's Sora and Ventus' nobody. He also looks like Ventus.
Organization XIII was formed by a group of scientists working under a man called Ansem the Wise, the ruler of Radiant Garden, and worked from Ansem's castle, Hollow Bastion. The head of this group was Ansem's apprentice, Xehanort, with the others being Braig, Dilan, Even, Aeleus, and Ienzo. Ansem the Wise began initiating tests about the human heart, focusing mainly on Xehanort, with the hopes of recovering his lost memories. At the urging of Ienzo, he constructed a massive laboratory under the castle. However, Ansem the Wise soon ceased his experiments, for he feared that going too deep would have grave repercussions, also noting Xehanort's "superhuman" abilities.
However, his six apprentices, still seduced by the desire for knowledge, conducted their own, dangerous, experiments on countless subjects. Eventually, they discovered the beings called Heartless, those that are born from the darkness in a heart and seek others as well. Xehanort, having taken on Ansem's name and writing his findings under it, opened a door he discovered, breaking down the barriers between worlds and allowing travel between them. And when the king named Mickey arrived, the six men's desire for knowledge eventually caused Xehanort to abandon his vessel, becoming a Heartless (though he was unusual in that he had a human form and retained his memories), with the other five soon following. Thus, the first six members of Organization XIII were born, and they banished Ansem to a realm of nothingness. Xemnas rounded up these nobodies, hook them up to a fake Kingdom Hearts and fill them all with Xehanort's heart and mind.
Roxas has no memory of his previous life because Sora didn't spend that much time as a Heartless in Hollow Bastion. While Roxas was in the organization he and Axel became friends but he left to find out why he had the Keyblade. Roxas is friends with Xion who is a replica of Sora. Xion later died. Roxas then met Riku who battles him for avenging Xion and defeats him. After Riku defeated him he took him to DiZ. DiZ then threw Roxas in a fake Twilight Town so the Organization couldn't find him. Roxas' friends in Twilight Town were Hayner, Pence, and Olette. He lives a good life until Axel breaks into the fake world to save Roxas.
When Roxas reaches the mansion in Twilight Town he gains his memories of the organization after meeting Namine. Roxas then battled Axel as he got deeper in the mansion eventually defeating him. Roxas then got to the part of the mansion where Sora, Donald, and Goofy were sleeping. Roxas reached Sora's chamber and he merged back with him which led to Sora being released from the pod he was in. The deaths of the Nobodies and Heartlesses lead to the original bodies ressurect.
Nobodies are born when a heart is swallowed by darkness and becomes a Heartless. They are the remaining parts left behind by the heart: the body, giving a Nobody form, and the soul, giving the Nobody life. However, only those with strong wills such as Xehanort/Terra and Sora are actually able to continue on as Nobodies. Those with the strongest wills manage to retain human forms with some slight changes to their appearance, while the rest possess malformed, barely humanoid appearances. Therefore, the more humanoid a Nobody, the more powerful it is.
All Nobodies, vanishing from the realm of light upon the loss of the heart, are born in a world in-between the realms of light and darkness, like Twilight Town or Castle Oblivion. However, because of this and their lack of hearts, they are shunned by both light and darkness, and do not truly exist.
Because of their missing hearts, Nobodies lack emotion. Although, some humanoid Nobodies pretend to possess emotions, fooling others as well as themselves. On the other hand, Nobodies manage to keep their memories of their former lives, but unusual Nobodies lack memories of their former selves. Such memories are the reason why they can pretend to have emotions. However, even though Nobodies don't have emotions, they can think and plan (as opposed to the Heartless, which simply act on instinct, with a few exceptions). Although it's never stated, the events of the Battle of 1000 Heartless implied that Nobodies are stronger than Heartless, since the Nobodies were able to easily overpower the Heartless. The death of a person's heartless and nobody leads to the return of the original person.
A common thread between all of the members is the "X" that appears within each of their names. The X idea was made because Xehanort wanted the X-Blade. By removing the "X", the remaining letters form an anagram of the members' original name from when they were humans. For instance, "Roxas" is an anagram of "Sora", and "Xemnas" is an anagram of "Ansem". Xemnas' true name is Xehanort; his name is an anagram of 'no heart' or 'another' with an X added. Such thoughts made many fans anxious to try and figure out for themselves the rest of the Organization's member's names. The name of the Nobodies' "other" appears before them in translucent, rainbow colored font (similar to the one shown at the intro to "The World That Never Was"). Then, an "x" drops down and the letters re-arrange themselves until they form a new name. It is also suggested that Xemnas is the one who performs this ceremony, although that may not necessarily be the case as Xigbar found Marluxia and quite possibly named him.
Each member in the Organization dons the same uniform, regardless of whether they are male or female. The uniform consists of a single black leather hooded cloak. The uniform was invented by Master Xehanort as a substitute for Keyblade armor. It is plainly adorned with a silver zipper and seemingly pointless silver beaded pull strings for the hood that form a semi-circle in the middle. Each member wears their zippers at differing points.
Not much is known about the under garments of the Organization. It is known, however, that they all wear hakama-esque black pants and knee high silver trimmed boots (although Roxas wears shoes of the same color, and Larxene wears somewhat of a female version of the boots with a heel). In addition to this, they all bear black leather gloves of some unknown length, as they disappear under bell shaped sleeves. Some members have different-looking coats, for example Axel, whose sleeves are tight around his wrists. Upon closer inspection, it can be seen that the hems of each cloak are each slightly different, as well as the shoulder pads (such as those of Demyx and Xigbar, which seem to be slightly pointed).
In the central tower of The Castle That Never Was, the room is something of a graveyard or a place where beings who can't use the portal to darkness use the doorway-like portals to reach other members of the organization. This chamber has thirteen portals which lead to a certain area throughout the castle, the portals are placed simultaneously along the walls and glow blue if that specific member is alive but red when that one is dead. Starting from Xemnas's which is actually a path to the The Altar of Naught, going downwards are two more then below them are four and after them are eight with a path cutting half way between the portals. The portals lead to large chambers which are areas that the Organization dwel when they are resting, so far only three are known with the Altar of Naught being were Xemnas resides and the highest point in the castle, Havoc's Divide which is a large slanted balcony were Luxord resides and Addled Impasse where Saïx lives which is a large circular room with a balcony made of glass and looks up towards the moon. Note that for gameplay purposes, the gates to Havoc's Divide and Addled Impasse remain active even after Luxord and Saïx are felled. Also, for the same reasons, Roxas' still-active gate (since he still exists within Sora) is broken and rendered unusable. It may be broken, though, due to the fact that while Roxas still exists, he does not physically. It is a theory that an organization member may destory this tomestones so that the fallen comrades secrets are not revealed to outsiders, and the only reason Xemnas did not destroy Saïx or Luxord's tomestone is because he was too busy with Kingdom Hearts, saying this Luxord or Saïx had amply time to destroy Xigbar's while Sora and co. where climbing the castle. The rooms name is proof that Nobodies tried to prove they exist. The tombstones, titles and portals may also be the Nobodies way of convincing themselves they existed or exist.