The first 34 quests of Kingdom Hearts Union Cross covered the first half of the game's introduction. After creating their avatar and selecting their Union, the player met Chirithy, an entity assigned to be their guide by Master Ava, their Union Leader. Ava tasked the player with collecting particles of light called Lux and weeding out traitors, and the player started their mission by setting off to new worlds, specifically, Dwarf Woodlands, Wonderland, and Agrabah. Here, the player met characters that shouldn't exist in the timeline they live in, which is the Keyblade War era — though, the fact that these characters shouldn't exist is not something the player's avatar is privy to.
Quests 35 picks up right after the player receives the Power Bangle from Nightmare Chirithy. This quest, along with every quest through 92, continues Union Cross' introduction and goes over details concerning the Foretellers and the Book of Prophecies. Throughout these quests, the player meets even more characters that shouldn't exist in Union Cross' time, and they're a lot more important the Kingdom Hearts series than the ones who've appeared so far.
There are many details in Kingdom Hearts Union Cross that can be missed or misinterpreted during a playthrough, so the purpose of this writing will be to both recap the game's events and explain its more confusing elements as they occur. For that reason, the portions of the story that could be considered filler will be included for context but also condensed so that the game's major plot elements can have more emphasis. Spoilers ahead.
Quest 35 - 81: The Main Disney Trio
In this questline, the player meets Mickey, Donald, and Goofy after the latter two crash-land in Daybreak Town's Waterfront Park. The crash destroyed the duo's Gummi Ship, and they requested the players' help in collecting the Gummi Blocks necessary to rebuild it. The player accepted and traveled between Daybreak Town, Dwarf Woodlands, and Agrabah to destroy the Heartless that made off with some of the ship's Gummi Blocks.
Mickey, Chip, and Dale show up halfway into the questline via Mickey's Star Shard to search for Donald and Goofy. Once they learn what happened from the player, they assist in rebuilding the ship. In the end, the player collects all of the Gummi Blocks, and all five Disney characters say their thank-you's and goodbyes before heading back to their world.
Overall, this questline doesn't have a significant impact on the overall story, but it does serve two purposes. It further emphasizes that there's something not right with Union Cross' timeline, as characters like Mickey and his friends, and technology like the Gummi Ship, shouldn't exist yet. This questline is also the first time that the character Luxu makes an appearance in Union Cross. When Mickey first shows up in Daybreak Town, he has the player escort Chip and Dale to Donald and Goofy. He stays behind to investigate why the Star Shard really brought him Daybreak Town. While he's contemplating this, Luxu spies on him from above.
Luxu is the sixth apprentice of the Master of Masters. By the time he makes his appearance here, the Master has disappeared, but before he vanished, he tasked Luxu with observing how events in Daybreak Town played out once he was gone. To Luxu's fellow apprentices, the Union Leaders, Luxu seemingly disappeared at the same time as the Master of Masters, but in reality, he's been skulking around in plain sight, observing. His spying on Mickey is the first time he appears in-game, but it's a super brief cameo, that only shows his legs.
Quest 82 - 94: A Chat With Chirithy
Sometime after Mickey, Donald, and Goofy's departure, Chirithy calls on the player to have a chat in Daybreak Town's Fountain Square. It believes that the player has become powerful enough to learn about the Book of Prophecies and the Foretellers who possess it. According to Chirithy, it and others of its kind were created by the Master of Masters. The Master had six apprentices, whom he gave names to after they became fully realized Keyblade Masters. He gave five of these apprentices each their own copy of the Book of Prophecies. The book is a tome that holds records of both past and future events. The apprentices who received a copy of it also took the moniker "Foreteller," referencing that they now know the future.
Upon reading the book's final entry, the Foretellers learned something shocking: a great war would occur soon, and it would end with darkness swallowing the world. In an effort to change this destiny, the five Foretellers devised a plan involving collecting light to use against the impending darkness.
They went about this by each creating their own personal group of Keyblade wielders called Unions. They then called on some of the many powers that the Book of Prophecies possesses, one of which was the creation of Medals, the trinkets that allow their wielders to channel the powers and abilities of people from the future (Sora's time period) and use them as their own.
Chirithy then confesses that while the Foretellers all have the same goal, they have different beliefs about how they can achieve it, so it warns the player not to give up their ideals in favor of their Union's. Now armed with the understanding of why they're collecting Lux, the player ventures out to collect it with more determination than before.
Disney Characters Explained
In its explanation of the Book of Prophecies, Chirithy didn't tell the player all of the ways the Foretellers use it because that knowledge is forbidden for Union members. These secrets are indirectly revealed in later quests, but as they also explain how Disney characters such as Mickey, Donald, and Goofy exist in the Union Cross timeline.
Simply put, none of the Disney characters that the player meets throughout their journey are real. Neither are any of the Disney-based worlds that the player visits. All of them are tangible illusions projected into reality by the Book of Prophecies. The book can take what it knows about the future, be it characters or worlds, and create tangible illusions based on them. Even all of the Emblem Heartless that the player fights aren't real, as they don't exist until Xehanort creates them in the ten years between Birth by Sleep and KH1.
With that in mind, Mickey never found out why the Star Shard brought him to Daybreak Town. If he did, he would have learned about the Book of Prophecies and the fact that he's just an illusion. The only reason he wanted to find out in the first place is becasue the real King Mickey would have, and the book is powerful enough that all of its illusions can pose as their real counterparts. Also, the reason the illusionary Mickey, Donald, and Goofy aren't confined to their own world like other Disney characters, is because their real counterparts actually have the ability to travel to other worlds, so they can too.
Kingdom Hearts Union Cross is on iOS and Android.