KH: Revised Dream and Reality Coincide Storyline

Allyson aka Ally

She’s from a world that got consumed by the darkness. Her father and her get separated from her mother and her twin brother Nathan (they are still MIA - presumed dead). Ally and her father end up in Radiant Garden/Hollow Bastion

Ally has the unique ability to sense the shifting tides between the balance of light and dark. She experiences this gift through seeing visions of KH events in her dreams – starting with the events that happen with Aqua Ventus and Terra – when she's about four or five. Ally tells her father about these dreams, and they consult the resident expert: Ansem the Wise.
After consulting him and learning a bit about other worlds - her father signs on to work with Ansem, as her dad's naturally interested in Ansem's work.

Then along the way - Xehanort "kills" (disposes of) Ansem, and he goes into the Realm of Darkness and Xehanort recruits the workers under Ansem, and they all destroy themselves; however, Ansem pushed Ally's dad away so he didn't get involved in feeling Xehanort's wrath.  Ally's dad gets extremely invested after Ansem's disappearance (and Xehanort and his crew) and picks up where Ansem left off – researching. Time passes and Ally begins to forget she ever had those strange dreams - until they appear again, but this time they star Sora when he begins adventuring (Ally's about 14 - same age as Riku).  

But before the dreams, Maleficent & darkness take over Radiant Garden/Hollow Bastion and make it unlivable, and Ally and her father (along with a friend named Saya) end up in Traverse Town (though not really associating with Leon and co.).

Unlike before, with her dreams about the Birth By Sleep crew, Ally resonates with Sora (due to the power of his heart and his bonds of friendship) so Ally remembers these dreams vividly and feels the need to find out more, more than she did when she was younger (also cuz she was like 5, so she didn't really have enough energy to invest or think further beyond the events of Ven Terra and Aqua being more than dreams).

Ally consults her father (along with her friend Saya - who's attuned with the magical/supernatural - and they discover that she's seeing true events unfolding outside their world, and revealing to them that there's danger lurking.

Then the Heartless appear and Ally's father is consumed by darkness (aka turns into a Heartless) - later down the line, across the realms, Ansem the Wise (not Xehanort's Heartless pretending to be Ansem) finds Ally's father's Nobody. However, not knowing how to restore him (as Ansem didn't know if her dad's Heartless was destroyed yet), he lets Ally's father reside within his own heart (so Ansem carries Ally's father with him, while currently going by Diz). Ally (and Saya) learn of her father's demise, and find ways in Traverse Town to recover him - but come up with nothing (despite even consulting Merlin the Wizard); though during this time, Ally and Saya never come across Sora Donald and Goofy cuz irony and fate, despite Ally continuing to have the dreams of Sora and his adventures continuing (Ally knew of Sora, but Sora knew nothing of Ally).

Once the events of KH1 end, Hollow Bastion is livable again and so Ally and Saya, with many of the residents in Traverse Town, return to the place they began to call their home. The events of 358/2 Days and Chain of Memories occur, and Ally's dreams are more intense than ever. And this time, she sees both Sora and Riku's excursions in Castle Oblivion. Ally also has visions of Roxas, but they are much more distorted due to his tie to darkness, and only being half of Sora (she never learns his name). Despite the dreams of Roxas being blurry and unclear at points, due to Sora needing his memories to be fixed in CoM, Ally (and Saya) begin to forget about Sora, and so Ally's time is spent wondering about this new boy in her dreams.

Ally, having enough of seeing what's going on around her and not doing anything about it, decides to take action and consult Merlin the Wizard. Merlin (with the help of Fairy Godmother) gives Ally training, in order to prepare her for the adventure to come. During this time, Kingdom Hearts calls out to her. Kingdom Hearts (in my canon) is a sentient being, and 'he' (he talks to her in a masculine voice, so Ally refers to him as a "he") seeing the power of her heart, decides to make her worthy of protecting the Keyblade wielders, though only specifies Sora.

In order for Ally to do well on her journey, Kingdom Hearts bestows Ally with a blade known as the Shattering Illumina. It has the ability for Ally to tap into the powers of light (and later darkness - though she doesn't know that to start off) and defend those important to her. Around this time, Sora is almost ready to be awakened. Before Sora is awakened, Kingdom Hearts helps Ally to travel to Twilight Town (the real one, not the digital one). Here, Ally finds Mickey cloaked in the Organization hood, and remembers Mickey from her visions (where she first saw him as Mickey, not "His Majesty"). Mickey is confused at first, but Kingdom Hearts intervenes and allows for Mickey to get caught up. Mickey then instructs her to go find a cloaked figure in the abandoned mansion on the outskirts of the town.

Ally listens to Mickey and goes to the mansion, tracked down by Nobodies along the way (primarily Dusks) and she doesn't know how to fight them (as Merlin and Fairy Godmother taught her how to counteract Heartless, and even equipped with powers from Kingdom Hearts, Ally is still ill-equipped to handle them).

When she makes it inside the mansion she sees a cloaked man, and following Mickey's instructions Ally follows him. She encounters the cloaked man and Diz, and she tells them her story. Diz, though curious about her predicament, doesn't realize it immediately the role Ally would play as she moves forward (as he can't talk to Kingdom Hearts). However, Diz/Ansem does know that Ally is the daughter of the man residing in him, despite not laying eyes on her in years - Ally's father from Diz's heart awakens and signals to his host that his presumptions of Ally are true.

In order to speed things along, he allows for Ally to meet Roxas, though in Roxas' digital form, as Ally talked to them about how she was having dreams about him (and indirectly Sora, though she can't remember currently, though she recognizes him and her memories flood back when she sees Sora in the pod).

Ally confronts Roxas and helps to convince him to return to Sora, and promises in return for Roxas' cooperation that Ally will remember him and help him live through her memories. Then the story unfolds with Ally accompanying Sora on his adventures with Donald and Goofy once they're awakened.