Axel, the , is Rank VIII within Organization XIII. He wields the element of flame and enchants his weapons, twin chakrams, with his element to unleash powerful rapid attacks. He commands the Assassin Nobodies.
Considered a Trump card by the rest of the Organization, he acted as Xemnas's double agent in Castle Oblivion, keeping track of Marluxia's plans for the other members, then betrayed the Organization for his friendship with Roxas, acting of his own accord to try and see him again once Roxas joins with Sora. Axel appears in Kingdom Hearts 358/2 Days as a playable character.
Axel is the Nobody of Lea, which was revealed in the Day 356 Secret Report and also mentioned by Saïx. It is highly theorized that this man had a connection to Saïx's original persona.
Before becoming a Nobody, the character we know as Axel was a man named Lea (pronounced "Lee"). Lea was friends with Saïx's original persona, the two planning to take over Organization XIII together. Once Lea joined, his name changed to Axel, Saïx's original persona's name changed to the latter. Identities weren't the only things that changed, however, as Saïx changed to an emotionless, calm Nobody in terms of personality, Axel's staying the same. This caused the bond Axel and Saïx shared as humans to break. Over time, Axel possibly forgot about their human selves' plans or chose to keep them secret and not think about them, as Saïx became Xemnas' second-in-command. He was later reminded by his old friend, however, after the creation of Xion, Saïx stating the lines that would reveal to the public for the first time that they had, in fact, been allied at one point, along with Axel's true name. Lea and Saïx's original persona's friendship later turned into a hostility, as seen in Kingdom Hearts II.
Axel's personality is very different compared to most other members of the Organization. His attitude also changes during the series. When he is first introduced at the beginning of Kingdom Hearts Chain of Memories, Axel has a mysterious, stoic, and calm demeanor on par with that of Marluxia. He seems to be a very good liar, as he earned Vexen's, Marluxia's, and even the trust of Larxene in only a short amount of time. He also seems to be very committed to the Organization, shown in a cutscene where he battles Marluxia. He seems quite distant at times during his time in Castle Oblivion, ignoring Vexen's insults and almost completely unfazed by Larxene's flirtatious behavior towards him. While he is seen as collected, intelligent, and even somewhat overconfident, cutscenes from Kingdom Hearts Re:Chain of Memories make him sound as if he is acting at times, possibly hinting at his dislike for the other members. Although in Re:Chain of Memories, Axel is shown to hold Larxene and Zexion in good company to an extent. Cutscenes show Axel making friendly conversation with Larxene while Marluxia is not around, and making friendly bets with Zexion as to which member will be defeated next. But eventually Axel betrays Larxene by letting Naminé escape and seemed disappointed yet satisfied upon her elimination. Axel also has Riku Replica annihilate Zexion to ensure that his actions in Castle Oblivion go unnoticed by the rest of the Organization.
Axel's personality changes drastically in between the two games, possibly due to his friendship with Roxas. In Kingdom Hearts II, he has a very laid-back, "go with the flow" outlook on many things, including missions and battles. But he also is shown to have a very kind, heartful side, ironic because he is a Nobody and has no heart. His friendship with Roxas and Xion and his moving final words are examples of this. He also stated that Roxas made him feel "like he had a heart".
In both games Axel has been shown to have a very typical "Nobody" side to his personality as well, being cold, unforgiving, and merciless. In Chain of Memories, he interrupts Vexen's cries for mercy with a cocky "Goodbye" before incinerating him without any hesitation. He also seemed to be amused by the deaths of his comrades as shown in the scene where he is making bets with Zexion. This cruel attitude also occurs when Marluxia uses Naminé as a human shield against him—Axel confidently declared he would kill Naminé to get to Marluxia. Also in Kingdom Hearts II: Final Mix he appeared indifferent to angering Xaldin.
In both Re:Chain of Memories and Kingdom Hearts II, Axel displays emotion, which is unusual given that Nobodies cannot feel anything. He himself notes this in Re:Chain of Memories when he begins laughing, but catches himself and stops. He even mocks his ability to feel towards his death. Whether he, Roxas, Naminé, and Xion all seem have something unusual in common regarding the heart, is unknown. But it has been hinted by Xemnas, and Axel himself, that by being close in contact with Sora's heart, Axel began to feel emotions of his own.
Out of the whole Organization, Axel is probably the most difficult to read. One never knows what he is thinking (as Lexaeus implied) or whose side he is really on. Most of the time he seems to be just messing around, but he may have done more thinking than everyone else combined, as stated in Jiminy's Journal.