Roxas is Sora's Nobody and a former member of Organization XIII, born when Sora took his own heart. Although Sora's heart was soon restored, Roxas continued to endure and exist, oblivious of Sora's existence for most of his life. Prior to the events of Kingdom Hearts II Roxas was Rank XIII within the Organization, and bore the title Key of Destiny because of his ability to wield the Keyblade. He wields the element of Light, and uses it to direct rays that strike opponents with great power. He commands the Samurai Nobodies.
Roxas was also the mysterious hooded figure in "Another side, Another story..." as well as in Deep Dive. During the prologue of Kingdom Hearts II, the player controls Roxas until Sora is awakened. In Kingdom Hearts 358/2 Days, Roxas is the main character. This game focuses on the time period between Roxas' birth and his departure from the Organization, eventually leading to his capture at the hands of Riku and DiZ.
Roxas, unlike other Nobodies, seems to possess real emotions, stemming from the fact that he, like Naminé, is a special Nobody. Many ranges of feelings come from him depending on his situation. Being Sora's other half, his personality is similar to that of his counterpart, with some differences.
For example, he is far more prone to anger than Sora; temperamental, aggressive, and stands his ground, characteristics that had mixed in his fighting style. He can be arrogant, confident, and determined as proven in his battles, more so when he engages Riku into a fight, proclaiming that he could not beat him no matter how many times he attempted to. His words have bite, and he isn't afraid to speak his mind. He even expressed some dislike for being compared to Sora early on due to their relation, which DiZ notes with amusement in their encounter. This type of main contrast between the boys is sometimes even portrayed in official art, with Roxas almost always having a dark, sort of fierce expression, while Sora's a bit more loose. With a devil-may-care attitude in terms of recklessness, he can be rightfully selfish in certain intervals, but not selfish enough to not, in the end, return back to Sora willingly in understanding. Throughout the game, Roxas is presented as the type to not just let things happen, to twiddle his thumbs and wait when there's something about himself he doesn't comprehend. He constantly rebels against what he's told is his "place" and is a strongly independent individual. On the other hand, Roxas tends to be ironically melancholy and stoic, solemn and distracted, especially as he comes to realize the truth about himself, and in other points when he's confused and lost. While he is also not as impulsive, like Sora, they both have little patience for people who seem to be bending their will or trying to confuse them. He doesn't have a extreme lust for life as well, but he does wish and desire one of his own. And despite their differences, the traits he and Sora share are shown in how they deeply care for their friends, along with some instances where Roxas is happy, cheerful, kind, at ease, and carefree in good company - even shown laughing.
Roxas also seems to be noted as the smarter half of Sora, as he is not as easily fooled by his enemies and is more likely to think his actions through unless angered.
Another difference than Sora is their heroic status, often times Roxas shows of how he'd rather not have been chosen by the Keyblade while Sora shows signs of enjoying his hero status with the Keyblade.