Keeping up with the lore of Kingdom Hearts is like doing a puzzle without a reference; the pieces all fit together, but there will be more questions than answers until the entire puzzle is completed. One such example is when players noticed that Roxas looks more similar to Ventus than Sora. The mystery is deepened as Roxas is Sora's Nobody, a being who lost their heart to the darkness. Nobody's typically retain the person's appearances whose heart succumbed to the darkness, but in Roxas' case, there was another factor that kept him from looking like Sora.
While Roxas is introduced before Ventus in Kingdom Hearts II and is the first of the two to appear in the canon, the story actually begins in the prequel title, Kingdom Hearts: Birth by Sleep. This title serves as an origin for Master Xehanort's evil plan to awaken Kingdom Hearts and follows his manipulation of the game's three heroes Terra, Ventus and Aqua. Ventus was the youngest of the trio and was used by Xehanort to try and create the χ-blade and open Kingdom Hearts.
Ventus' refusal to participate in his schemes and use his darkness to defeat enemies forced the villain's hand, and Xehanort extracted the darkness out of Ventus' heart. That residual darkness was then used to create Vanitas, a dark mirror and rival of Ventus, forcing the young hero to fall comatose following the ordeal. With his heart fractured and slowly fading, what was left of Ventus' heart travels to Destiny Islands, where it connects with a young Sora, allowing it to heal. In time, Ventus regains his consciousness but loses his memories following the encounter. His story continues through the events of Birth by Sleep, but the hero is dealt a blast from the past by the game's conclusion.
Ventus' story in Birth by Sleep concludes with the hero fighting Vanitas one more time for control of his body and in a last-ditch effort to stop the forging of the χ-blade once again at the cost of his heart. With Ventus victorious, his memories returned and the blade destroyed, but the hero lost his heart and is rendered comatose. Following the battle, Aqua takes him to the chamber of waking to rest while his heart travels to Sora, once again, to reside in his body as it heals.
Sora's story officially begins in Kingdom Hearts, and throughout his adventures, he learns of the Heartless, the Darkness, and he gains his iconic weapon, the keyblade. In the game's climax, Sora learns that Kairi's heart resides within himself; without it, she can't awaken and fulfill her own destiny. To ensure Kairi's revival, Sora sacrifices his own heart and so that hers may return to her body. As a result, Sora briefly becomes a Heartless until Kairi uses one of her abilities as a Princess of Heart to return him to normal. However, in that brief moment where Sora is a Heartless, Roxas is born.
Because such a large part of Ventus' heart was still in Sora upon his sacrifice, the Nobody known as Roxas took on a large percentage of Ventus' characteristics instead of Sora's. This also allows him to feel emotions, a trait other Nobody's can't replicate. While there are slight variations in appearances, such as Roxas' hair color being darker than Ventus', the story behind the mystery further proves the importance of each connecting piece in Kingdom Hearts' story.