Roxas, was the thirteenth and newest member of the Organization, and was created in Twilight Town when Sora gave up his heart to save Kairi in the original Kingdom Hearts. Due to Sora returning to his original self after being turned in to a heartless, Roxas and Sora both co-existed. Also due to Sora returning to human form so quickly, Roxas does not retain any memories of his previous lifes (Sora's and Ven's lifes) unlike the other Organization members. He is a lookalike of Ventus since Ven's heart was in Sora's body.
While Roxas was wandering about Twilight Town after his creation, he was discovered by Xemnas, leader of Organization XIII (Organization XII at that time one would guess). Outside the gates to the mysterious mansion of Twilight Town, Xemnas gives Roxas his name. Like all Organization members, Roxas' name is an anagram of his original name with an X added. Xemnas invites Roxas to join the Organization so he could use him to harvest hearts with his keyblade. Roxas decides to join in order to find out about his past.
Upon entering the Organization, Roxas meets Axel, and they both find a companionship in one another that is rare amongst Nobodies. After completing missions, the two would meet at the top of Twilight Town's Clock Tower and have sea-salt ice cream, like they did on the first day that Roxas was "born."
After Axel left for Castle Oblivion, Roxas was in charge of the 14th member of the Organization, Xion, who could also wield the keyblade. Soon the two became fast friends. Afterwards, during a mission in Agrabah, Roxas collapsed and fell asleep for days, starting to have a dream of a boy with red shorts, and hearing girl's voice who said that she knew that he was "snoozing". He soon awake, to find seashells left by Xion, one for each day he slept. The two met again at the clock-tower in Twilight Town. Roxas takes one of the shells and places over his ear, where he heard the familiar sound of the crashing waves. Around this time, news of everyone at Castle Oblivion being destroyed spread. Roxas fears that Axel was one of them. Roxas was also had this odd experiences of seeing images of the boy in the same situations that he is. Soon Axel returns, and what Roxas wanted is to have sea-salt ice cream
together with Axel and Xion. But Xion was on a mission and didn't report back in a long time. Axel and Roxas team-up to find her, and help destroy the heartless she was sent to destroy. And then, the three finally had ice-cream together.
As time passes, Roxas becomes impatient and wants to know about his past. No one in the Organization would reveal to him where he came from and why he can wield the Keyblade. Roxas worked to help the Organization in their plan for Kingdom Hearts by killing heartless but eventually, Roxas becomes fed up with his questions going unanswered. With recurring dreams of Sora, the other self he didn't know, Roxas decides to leave the Organization to find Sora and find out about his past, hoping to find out why he has the power to wield the Keyblade. Axel, intercepts Roxas and tries to convince him not to leave Organization XIII, warning him that the Organization would come after him if he left. Roxas ignores Axel and leaves him behind, leaving Axel distraught for his loss of a good friend. With nowhere to go, Roxas went to Twilight Town, on top of the Clock Tower, where he was confronted by Xion and learned the truth about her. Being re-programmed by the Organization, Xion attacks Roxas, in which she can absorb him and become "Sora." In order to defend himself, Roxas had to destory Xion. As she was fading away, in both her "body" and the memories of her, she returns back to Sora, and informs Roxas of Xemnas' plan for Kingdom Hearts and that Roxas must free them. That, and that both Roxas and Axel were her best friends. Right before she disappeared entirely, Roxas manage to remember her, making him shed a tear for the lost of his friend. At that moment, he manages to wield Xion's keyblade, which becomes Oblivion, along with his own, Oath Keeper, at the same time.
With Xion gone, Roxas returned back to the World that Never Was, intent of freeing Kingdom Hearts, just as he promised Xion, and become one with Sora. Before he can do this, Roxas is attacked at Memory's Skyscraper by a swarm of Heartless. Roxas tries to fight of all the heartless but realizes that there are too many of them. He looks up at the skyscraper to see a blindfolded Riku at the top. Roxas runs up the wall of Memory's Skyscraper, being chased by heartless, and tosses Riku his Oblivion keyblade to fight with. After defeating all the heartless, they quickly turn on each other. Riku was aware of Roxas's plan, and needed to stop him. Otherwise, he would be destroyed by the Organization, along with Sora's memories. Around this time, both Roxas and Riku were forgetting Xion, as Roxas barely recalls her name. Since Riku was standing in Roxas' way, the two fight. As they battle, Riku questions why Roxas has the Keyblade and asking "What's wrong Sora? Giving up already?" Though Riku manages to take him down, Roxas springs back to his feet and defeats Riku. Realizing that, as he is fighting in the Realm of
Darkness, he must embrace himself in that very same darkness to fight to his true potential. Riku opens his heart to Darkness, and in the process takes on the appearance of the Darkness residing in his heart (Xehenort's Heartless). With his new found strength, Riku knocks Roxas unconscious, with that; he last memories of Xion faded away completely. As he laid there, Roxas hears Xion's voice telling him "Don't be sad. I came from you and Sora. I am you, just as I am Sora. You'll forget me, the memories themselves will never go away. Memories of you and me will always be together, forever, inside him." DiZ suddenly appears over Roxas' body and Riku and him take Roxas back to the Twilight Town mansion. DiZ places Roxas in a digital recreation of Twilight Town so that Organization XIII and the nobodies will not find him. There, Roxas would be kept safely away from the Organization while he himself lived unaware of his former life and his nature as a Nobody.
Roxas goes on to live a normal life in the digital Twilight Town for some time, and befriends three other teenagers, Pence, Olette and Hayner (who's relationship with Roxas is similar to Riku and Sora's friendly rivalry at Destiny Islands). As Roxas sleeps, he dreams of Sora and his adventures (which serves as a refresher to the player, as Roxas' life in Digital Twilight Town serves as the Prologue to Kingdom Hearts II). What seems like a simple recurring dream is actually Sora's memories returning (at the hand of Namine). At this point, strange events begin to occur around the town (mainly thefts), and cryptic white creatures (Dusk Nobodies) begin to invade the town.
This invasion is the result of Organization XIII's discovery of the Digital Twilight Town, and after they sent the Dusks in, they sent Organization Member VIII and old friend of Roxas - Axel there, ordering him to either get him to return, or destroy him. As well as the Nobodies and Axel, Roxas comes into contact with Namine several times, who Roxas soon realizes she knows more about him than he does. Also, as observed by DiZ, Namine and Roxas' encounters connected Roxas' heart to Kairi's, who Roxas remembers as a friend of Sora. Roxas also meets a mysterious man in a black cloak (who would later be revealed to be Riku), who at one point goes as far as to enigmatically ask "Do you feel Sora?", and lastly Roxas meets DiZ, perhaps the most mysterious character of all. DiZ is a exceptionally wise man, who, unbeknownst to Roxas (and the player at the time) is responsible for Roxas' imprisonment.
Even more mysterious than these characters, Roxas also begins to witness strange anomalies, such as time freezing (with only himself and a few other people unaffected) as well as the appearance of several 'glitches' around the town (which Roxas and friends attempt to find, although they are fixed by DiZ before they are discovered). Things would soon take a dramatic turn however, when it was revealed to Roxas by Namine that he was a Nobody and that he is expected to merge with Sora sometime soon. Roxas explores a mysterious mansion found outside of Twilight Town, and discovers that he is living in a digital Twilight Town, and that his friends and memories aren't real, but rather just data. He makes his final encounter with Axel when Axel is ordered to eliminate Roxas due to his unwillingness to return to the Organization. Axel, who was previously overjoyed when Roxas began to remember his name, was seen to be visibly angry when Roxas told him he remembered him now, telling Roxas that its too late now. Axel unleashed his fury on Roxas, but was defeated by a dual-Keyblade wielding Roxas, who had just awakened all of his dormant abilities and powers.
After his battle with Axel, Roxas finds the room containing Sora's pod, and after some dialogue between Roxas and a hologram of DiZ, Sora's pod opens, and the two merge, thus making Sora complete once again.