The protagonist of Kingdom Hearts Union Cross is a customizable avatar created by the player. Usually, customizable avatars don't have a personality of their own because they're supposed to reflect the player's. However, the Union Cross protagonist defies this rule—if only a little bit. While they do follow the silent protagonist trope, they also have something of their own personality, which they express a little of this personality for the first time in the questlines set between quest 95 and 250. These quests also show what happens to Chirithy when their wielders are destroyed. This is a recap and explanation of these events, so there will be spoilers ahead.
In the previous recap, the protagonist met Mickey, Donald, and Goofy, and helped the trio rebuild their destroyed Gummi Ship. The recap also explained how Disney characters such as Mickey, Donald, and Goofy exist in Kingdom Hearts Union Cross' timeline. Following their meeting with the Disney trio, Chirithy invited the protagonist to talk. Believing the protagonist to be powerful enough to handle a few truths, Chirithy revealed who its creator is, who the Foretellers are, and spoke a little about the Book of Prophecies. It also revealed the critical reason behind why the protagonist needs to collect Lux, and so the protagonist sets out to collect more; quest 95 picks up immediately after this revelation.
Quest 95 - 230: Olympus Coliseum Introduction, Disney World Revisits
Several things happen during this batch of quests. The player hunts down a new breed of Heartless called Darkballs and tracks their origin to Olympus Coliseum. This new world ropes them into a preliminary tournament to see if they're worthy of potentially becoming a hero. From here, they visit Agrabah and help Aladdin clear his name after he's suspected of stealing money. Then, in Dwarf Woodlands, they finally managed to reunite all the dwarfs before meeting Snow White for the first time. Finally, in Wonderland, they help escort Alice to the White Rabbit's house.
There are two notable moments to take away from these quests, the first being the Darkballs. The appearance of these Heartless is the first indication that the strength of the darkness is still growing despite the Keyblade wielders' efforts to fight it back. The reason behind this growth comes from the appearance of the Nightmare Chirithy, the one that gave the player the Power Bangle near the very beginning of Union Cross. The darkness will spike in strength again during the questline between 231 and 250, and when it does, people will start disappearing.
The second important moment is the reveal of the protagonist's internal struggle—the first sign that they have their own personality. At the end of the Dwarf Woodlands quests, the dwarfs agree to let Snow White use their house as a haven from Queen Grimhilde, and she cooks them their favorite meal to celebrate. The protagonist doesn't stay for the festivities, though, and Chirithy confronts them about it in confusion. Of course, the protagonist doesn't say anything, but what they're trying to convey can be interpreted from the way Chirithy responds.
Essentially, the protagonist feels alone and isolated. They don't have connections back home in Daybreak Town like the dwarfs have with each other and, now, Snow White. They would stay for the party, but they would essentially be a third wheel, as this isn't their world. Chirithy tries to cheer the protagonist up by saying that their friends are always with them, but the protagonist doesn't have any friends. They aren't part of a "Party" and don't personally know any other Keyblade wielders.
"Parties," during the Keyblade War era, were groups of Keyblade wielders from the same Union that banded together to tackle missions given to them by Chirthy and their Union leader. Union Cross as a game actually has a mechanic where players can create and join parties of their own, but doing so isn't canon to the actual story.
After assisting Alice in Wonderland, Chirithy informs the player that another breed of Heartless called Neoshadows has emerged, and it needs their help quelling them in Daybreak Town. The player portals back to their world and ends up in the Moogle's Shop, where they meet a group of fellow Keyblade wielders from their Union. These wielders are taking on the Neoshadows too, and after learning the protagonist is a Keyblade wielder, they want to work with them when they set out. The plan is to split up and clean up different parts of Daybreak Town. Once the Heartless are defeated, the group will meet back up in the Moogle Shop to celebrate. The player agrees, and the wielders set out on their mission.
Meanwhile, the protagonist's Chirithy meets up and talks with the Chirithy belonging to the leader of the Keyblade group the protagonist just met. The other Chirithy is concerned about its wielder. It thinks that they feel indifferent to the cause of gathering Lux to save the world, or that they don't understand what's going on. The player's Chirithy tries to reassure the other, telling it that its wielder will flourish soon enough.
Back with the protagonist, they team up with their new acquaintances a few times while destroying Neoshadows. However, as the day goes on, they run into their teammates less and less. Upon clearing the Heartless, the protagonist returns to the Moogle Shop as planned, but instead of meeting their new companions, the same Chirithy that was talking with the protagonist's earlier is waiting for them. It tells the protagonist that their new friends aren't coming, but they apologize for not keeping their promise. It then teleports away, leaving the protagonist sad and confused.
This same Chirithy returns to the 2nd District, where the player's Chirithy finds it collapsed on the floor. The player's Chirithy asks what's wrong, and the weakened Chirithy explains that the darkness took its wielder, meaning the Neoshadows destroyed him. The weakened Chirithy then wonders if it's going to die, and the player's confirms it, saying that all Chirithy are connected to their wielders, so if a wielder is destroyed, their Chirithy will disappear with them. The weakened Chirithy admits it understands this before it ceasing to exist.
The moment the weakened Chirithy dies and vanishes, Master Ava appears before the player's Chirithy. She's surprised at how fast the darkness is growing despite the rate at which the Unions are gathering Lux. The player's Chirithy wonders if that means the Foretellers really have a traitor among them, but Ava can't bring herself to believe that yet. With that, the questline ends.
To clarify, the Chirithy that dies is the same Chrithy that visits the player in the Moogle Shop. Some believe the player's Chrirhty was the one to deliver them the bad news, but that's incorrect. The player's Chirithy wouldn't know anything about the other wielders, and if it did, it wouldn't feel the need to ask the dying Chirithy what's wrong with it because it would know that Neoshadows destroyed its wielder and his friends.
Kingdom Hearts Union Cross is on IOS and Android.