According to the Secret Ansem Reports,Roxas was born when Sora stabbed himself with the Keyblade of People's Hearts forged from the hearts of six of the seven Princesses of Heart in order to free Kairi's heart at the cost of his own. However,Sora spent little time as a Heartless,as Kairi's light had fetched Sora's heart from the darkness,restoring him to his former self shortly after he first became a Heartless. 

Thus,Sora and Roxas co-existed,but Roxas was missing his memories,the memories of Sora,and the two were incomplete without one another. Still,Roxas's status as Sora's Nobody granted him the ability to use the Keyblade.This makes Roxas a very unusual Nobody,besides his capacity to experience actual human emotions, something that Nobodies are normally incapable of.However, as he wields a Keyblade,which serves as a homing beacon for the Heartless,he is constantly under risk of being attacked by them when not under the protection of other Organization members. 

Roxas and Xemnas talked along the bank of the Dark Realm, the Dark Meridian. This is where Xemnas said that he had been to see him (referring to Sora).This is referring to when he was a secret boss in Kingdom Hearts Final Mix. 

Roxas also appears to have a psychic link with Sora, as he begins to absorb Sora's memories as Naminé restores them and falls into a coma right when Sora was put to sleep.

He was born in Twilight Town and soon discovered by Xemnas, who gave him his name. Having nowhere else to go, Roxas joins the Organization in hopes of finding the answers he needs. On Roxas's fifth day, Xemnas places Roxas under Axel's watch, and the two soon become best friends. When he visits Twilight Town, sometimes he catches sight of three regular kids: Hayner, Pence, and Olette, and wishes to befriend them.

When Axel is sent to Castle Oblivion to interfere in Marluxia's plans to enslave Sora and overthrow Xemnas, Roxas is placed in charge of Xion, the newest member of the Organization and a girl about the same (physical) age as him. Though emotionally distant at first, Xion soon opens up to Roxas, revealing to him that she can wield a Keyblade, and the two become close friends after Roxas falls into a coma for several weeks, linked to Sora being placed in suspended animation by Naminé. After awakening, Roxas attempts to spend time with Axel and Xion, but he begins to react to the process of Sora's memory restoration, and begins to question why he wields the Keyblade and doubt the Organization's motives. At the time when he is at the Organization he begins a lot of missions involving visiting other worlds which cause Roxas to experience some of Sora's memories.

Knowing that Roxas would not be able to kill her, and Axel had admitted that he could not either in his own Secret Report, she forces Roxas to fight her instead. Xion then transforms into a giant armored monster at the ready. After a long and grueling battle, Roxas defeats Xion, who regains her original appearance. Roxas, however, loses all of his memories of Xion after the battle. In her final moments, she informs Roxas of the true colors of the Organization, tells him that she is glad they were friends and pleads with him not to let Xemnas accomplish Kingdom Hearts before dying peacefully in his arms. At this moment, Roxas remembers Xion, but he is too late. Saddened, Roxas holds Xion's essence and degenerating crystallized body until she is completely gone, leaving behind a seashell on the ground, as he spills tears.

This marks the start of where he gains his signature ability to dual-wield, as he awakens Ventus's Keyblade after her demise. The first time he uses them, it shows two Kingdom Keys, his and Ventus's, which quickly transform into the Oathkeeper and Oblivion.

In Kingdom Hearts 2,the hints of Roxas's presence within Sora come to a climax in the World That Never Was. At Memory's Skyscraper, two Samurai Nobodies arrive and block Sora from Donald and Goofy. Then, Roxas, shrouded in a black coat, confronts Sora, summoning his Keyblades (much to Sora's surprise). The two are transported to the Station of Serenity, and they share an intense battle, which was originally depicted in a short cutscene, but became an actual boss battle in Kingdom Hearts II Final Mix. During their fight, Roxas says that he is someone from the dark, and Sora wonders if he is Riku. His guess is incorrect, and Roxas says that he had defeated a Riku once. 

He also asks a strange question: "Tell me... Tell me why he picked you!", referring to the Keyblade (the confusion stems actually from a mistranslation in dialogue) and attacks Sora in a rage. At one point, Roxas floats from above, and sees the images of Riku, Kairi, Donald, and Goofy next to Sora, and finally understands that Sora has the Keyblade because of the bond with his friends. Still, Roxas continues to attack, and is able to knock the Keyblade out of Sora's hands. Sora, however, recovers the Keyblade and swiftly cuts a surprised Roxas down. Roxas, wounded, drops his weapons and his hood, revealing his face, and tells Sora, "You make a good other", disappearing with a satisfied look.

It is revealed that the encounter was actually a mental struggle between Roxas and Sora when Sora returns to the World That Never Was. He asks his companions about the mysterious Keyblade-wielding Nobody, but Donald and Goofy say that they saw no one other than themselves, and that Sora had just disappeared, leaving them to fight some Nobodies. Sora is confused, but continues on.

Roxas and Naminé are seen one last time, when Sora and Riku return to Destiny Islands. As Sora and Kairi reunite, they smile at each other, while Roxas and Naminé do the same. With the death of Xemnas, Roxas could be considered the only surviving member of Organization XIII, if only an ex-member.