When the original Foretellers still held power in Kingdom Hearts Union Cross, they guarded secrets that they considered forbidden knowledge to the wielders serving under them. The Chirithy knew at least some of these secrets, but they were, of course, prohibited from telling their assigned wielders anything. The Foretellers didn't withhold this information out of malice, but because they were told to do so by the Master of Masters.
Still, while they did a relatively excellent job of hiding the fact that there were secrets to learn, one Keyblade wielder managed to stumble upon evidence of their existence, and his natural curiosity led him to seek out their answers. This wielder's name was Ephemer, and he's probably the most focal hero character introduced in Kingdom Hearts Union Cross. Spoilers ahead.
Quest 251 - 350: Finale of Dwarf Woodlands & Olympus Coliseum
Following the Neoshadow massacre, Chirithy tries to take the protagonist's mind off of things by keeping them busy. In a quick visit to Agrabah, they help the citizens keep the town clear of Heartless. Afterward, they head to Dwarf Woodlands and Olympus Coliseum, where they end up finishing both world's storylines. In Dwarf Woodlands, Queen Grimehilde is killed following a battle with the protagonist, and the coma-like sleep she placed Snow White in is broken by a kiss from Prince Florian.
In Olympus Coliseum, the protagonist meets and defeats Cloud in the finals of a tournament. Then they team up with Cloudand Hercules to defeat Hades when it's revealed that he orchestrated the tournament as a scam to drag the winner into the Underworld. In a later event, the player battles against Sephiroth, who, after learning the protagonist knows Cloud, leaves and promises to meet again. With that, the protagonist finally returns to Daybreak Town to rest.
Quest 351 — 358: The Dream
Sleeping in their apartment, the protagonist has a very peculiar dream. They find themselves within Daybreak Town's clocktower, standing at the entrance of the Foreteller's Chambers. Deep within the room, they see the Master of Masters discussing something with the Foretellers, but they can't make out what's being said. They try to make their way further into the room, but the moment they do, darkness swallows up the dream, and they awake in a cold sweat to Chirithy watching over them.
Chirithy manages to coax the protagonist back to sleep, and once it does, it confronts a hidden entity that's nearby, asking it if it was the one who stopped it from showing the protagonist the dream. The imposter Chirithy responds by materializing outside of the apartment window and asking Chirithy what it meant to do by showing the protagonist the dream in the first place. Chirithy simply tells the imposter that it's doing the opposite of whatever it has planned, and so the imposter marks it as its enemy before disappearing. Chirithy then turns its attention back to the protagonist and promises to protect them.
The dream Chirithy nearly induced in the protagonist was supposed to show them moments from Kingdom Hearts Back Cover. The player can tell because what the protagonist manages to see matches up with a scene from the movie, the one where the Master of Masters tells the Foretellers to create their own Unions and gather Lux.
Chirithy once told the protagonist that the Foretellers argue over how to achieve their goal of saving the world, and it did that because it fears there's a traitor among the group, one planning to use their Unions to their advantage. Since it doesn't know who to suspect, it wants to reiterate to the protagonist that they need to follow their own heart before that of the Foretellers', and so it planned to show the protagonist how splintered the group was, same way Back Cover did.
Quest 359 — 375: Ephemer
The next morning, the protagonist is going about their daily routine of collecting Lux when the town Moogle waylays them. It warns that there's a powerful Invisible Heartless terrorizing Waterfront Park, so the protagonist rushes to meet the threat. When they reach the location, however, they witness the tail-end of a battle between the Invisible and Ephemer, which the latter narrowly manages to win. After the protagonist helps him to his feet, Ephemer officially introduces himself and claims to be from Master Gula's Leopardos Union.
For the first time ever, the protagonist responds by actually speaking, introducing themselves with whatever name the player has given them, and claiming that they're from Master Ava's Vulpes Union. Ephemer notes how the two of them are on different teams, but he's currently working on a personal project so the protagonist can consider him "Union-free." When the protagonist expresses curiosity, Ephemer decides it would be safe to tell them since they just helped him out.
Ephemer explains that he has a hunch. He believes that the different worlds Keyblade wielders travel to aren't real, just illusions created by the Book of Prophecies. The real versions of these worlds are connected to Daybreak Town by land but are impossible to reach, so the Foretellers used the book's powerful illusions instead. These illusions are indiscernible from their real counterparts, making it possible for wielders to collect Lux from more places than just Daybreak Town. However, he's recently started seeing wielders fight over the Lux instead of gathering it, which is concerning. He doesn't know why it's happening, but he thinks it has something to do with the Foretellers, as his digging has taught him that they all have different agendas.
This statement causes the protagonist to remember the dream that they had, which Ephemer finds interesting. He invites them to join him on his mission, and when they happily accept, the duo set out to look for the place the protagonist saw in their dream. After they've gone, Chirithy appears and once again confronts the concealed imposter, asking if Ephemer is its wielder. The imposter Chirithy materializes too, and upon seeing it in the daylight, Chirithy notices that its fur is darker than before. The imposter sarcastically applauds Chirithy's perceptive abilities before asking if it's just going to let Ephemer discover the truth.
Meanwhile, Ephemer and the protagonist find an underground passage system that will let them make their way into the clock tower. The two work together to defeat the Heartless infesting the passage, but when they take down another Invisible blocking their way, Ephemer calls it a day. It's taken them all day just to find a way into the clocktower, and they have no idea how much longer it'll take to reach the Foreteller's Chambers. At this rate, their Union leaders will get suspicious of them.
Ephemer then seemingly diverts from the topic and asks the protagonist if they too feel this little venture has made them friends. When the protagonist agrees and shakes Ephemer's hand, Ephemer suggests that they meet up at Fountain Square tomorrow, then come back. They know how to get in now, so it shouldn't take nearly as long. The protagonist agrees to the plan, and the two make their departure.
That night, Chirthy watches as the protagonist struggles to get to sleep. When it asks what's got them so excited, the protagonist explains that they made a friend. Chirithy is happy for them, but it doesn't have friends, so it's not familiar with the sentiment. To Chirithy's surprise, however, the protagonist refutes this and claims that they're its friend. Chirithy has never thought of their relationship like that before, but it finds the idea comforting.
Unbeknownst to the protagonist, however, Ephemer returned to the underground water tunnels that same night and regrettably continued without them. The protagonist waits for Ephemer in Fountain Square from sunrise to sunset, but he never shows up. Finally, when the night arrives, Chirithy appears, confused about why the protagonist hadn't gone with Ephemer yet. Once Chirithy understands the situation, it suggests the protagonist gives Ephemer the benefit of the doubt and asks them to cheer up because it gets sad whenever they are.
Chirithy concern lifts the protagonist's spirits, and to Chirithy's surprise, the protagonist adoringly scoops it up from the ground. The act reveals Chirithy is a Dream Eater, as its cape flaps up and reveals that it has a Dream Eater symbol branded on its back. With that, the questline then comes to a close.
Kingdom Hearts Union Cross is on IOS and Android.