(no subject)
* The story of Kingdom Hearts 1 follows canon almost perfectly, but there are differences here, too. Vanitas Sentiment appeared on the islands a few times, slowly coaxing Riku into the dark. In this universe, Xehanort made many plans to bring back his friends and reunite the worlds. As a part of these plans, Riku would wield the Keyblade, something Terra initially chose Riku to do, anyway. He also wished to eliminate light and darkness, having been forsaken by the so-called "balance".
* Vanitas Sentiment, however, intended to ruin these plans. As a true Unversed he has a strong disregard towards life and is far more 'evil' than he used to be (as Vanitas he at least possessed some amiable traits). Therefore, he stepped in to alter the grand scheme of things by aiding Riku's curiosity about other worlds and the darkness. All the while, "Ansem", the corrupted remains of Master Xehanort, also fueled the curiosity of both Sora and Riku.
* Canon continues from here. Everything in KH1 wraps up as planned, but when Sora frees Kairi's heart the remains of Ven's heart and consciousness merge with Sora's newly formed Nobody, creating Roxas. Roxas has all of Ven's memories suppressed in his fragmented heart, far from reach. Roxas is recruited into the Organization, but Xemnas' plans for him are different. He wants to use the Keyblade still, but there's an ulterior motive- Ventus.
* Xion, the 14th member, was created to aid in Ven's "awakening", as his body was removed from the Chamber of Waking by Vanitas. Essentially, she was to become Ven's new body. Unlike in canon, she took form from the memories resonating within Roxas, appearing to most people like Aqua, although others saw her either as a hooded doll or Ven himself. She and Axel befriend Roxas, each of them feeling...strangely familiar. While Days canon is similar to a degree, Xion's purpose is much, much different, and Sora's memories progress without any complications.
* While Xion did meet Riku, the circumstances were very different. DiZ saw her as an obstacle that needed to be destroyed, and so he sent Riku to eliminate her. However, he wound up having a change of heart after seeing her up close and especially upon noticing her similarities to Aqua, who he met as a child. During their meeting he also gave her some advice, similar to in canon.
* As time went on, she became stronger while Roxas grew weaker. Her form changed, too. Eventually she took on the appearance of Vanitas, and Roxas was forced to fight and destroy her. Upon absorbing Xion, however, something went wrong in his mind. Memories began to flood his consciousness. While in the past he'd had dreams of a boy in red and occasionally three strange Keyblade wielders, these memories were clearer.
* Seeking answers, he headed to look for Xemnas, all while hoping to keep his promise to set Kingdom Hearts free. Riku intercepted him along the way. Cue deep dive fight. Roxas was then captured, but not because of his importance to Sora. On the contrary, it's because DiZ didn't want the Organization to get their hands on such a useful tool.
* So as Sora's memory restoration finished up, Roxas was kept in the simulated Twilight Town. In order to halt his 'awakening', DiZ even created new memories and a fake personality in hopes of completely suppressing any traces of Ventus within him. While it worked for a while, the Organization made quite a few attempts to bring him back, while Roxas himself eventually grew restless. With the help of Namine, he was able to escape the simulated town and started to search for answers yet again.
* Unfortunately, his time was limited. In his travels he was intercepted by Vanitas Sentiment and later destroyed, his piece of Ven's heart used to help the Sentiment gain a physical form by using Ven's body. It worked, but only half way, as the other half of Ven's heart rested in Sora.