A lot is changing with the future of Kingdom Hearts, that much is made clear immediately with the latest trailer. During the 20th anniversary celebration of the series, Square Enix took the opportunity to finally unveil gameplay for the previously teased Kingdom Hearts 4. In it, players finally got their first substantial look at Quadratum, as well as Sora's fate after Kingdom Hearts 3's secret ending. Sora is sporting a new look alongside a seemingly new ally, as the Heartless invade the World of Fiction, and the spry Keyblade wielder shows off some brand-new moves to take down the Darkside Heartless. As for who this new girl is who visits Sora in Quadratum, some fans may have been surprised to see her.
Fans love to share memes that Tetsuya Nomura and the Kingdom Hearts creative team are always on to something cheeky, and Kingdom Hearts 4 has already proven that tenfold. "Quadratum" is Latin for "Square," with nearly every tease of the World of Fiction being a parallel to the canceled Final Fantasy Versus 13, among other "coincidences" like Kingdom Hearts 4 being the 16th installment, and the square root of 16 being four. However, the appearance of the crimson-haired Strelitzia in the Kingdom Hearts 4 trailer is actually one of the more straightforward teases from the trailer. Given Sora's fate until now, it makes sense he would meet up with Strelitzia in the next Kingdom Hearts game.
The Mysterious Red-Haired Girl: Strelitzia
The truth is that this new "mystery girl" isn't quite a mystery for hardcore Kingdom Hearts fans that are in the know, especially for those who have played the entirety of Kingdom Hearts Union Cross. Strelitzia was a Keyblade wielder from Daybreak Town who was around for the first Keyblade War, hundreds of years before the events of the mainline Kingdom Hearts games. She was among the several Keyblade wielders who would become "Dandelions," a secret collective of heroes who would rebuild the worlds of light after the Keyblade War. Alongside her brother Lauriam, Ephemer, Skuld, and Ventus, Strelitzia was to become a new Keyblade Union leader after the war.
However, she was apprehensive of becoming a Union leader because of her knowledge of the coming Keyblade War, and how many Keyblade wielders were fated to perish in the forthcoming conflict. In Union Cross, she was to inform the unknowing player character that this Keyblade War would mean a grim fate for many of their friends and allies, but she never gets her chance to speak to the player. Upon learning the player's last known whereabouts, she's ambushed in an abandoned building by a shadowy figure, and she loses her heart. Ventus eventually replaces her as Union leader, but he did not knowingly attack Strelitzia that day, and she's been gone since.
Strelitzia, Quadratum, and Sora
Given the context of Strelitzia's fate in Kingdom Hearts Union Cross, it makes a lot more sense that she's the one who finds Sora in Quadratum after Kingdom Hearts 3. Sora's use of the Power of Waking to bring back Kairi from losing her heart was a twist of fate, something that could not go unpunished. With Kairi returning to life, Sora disappears from their universe, and is sent to Quadratum as a result of some kind of equivalent exchange. Strelitzia is the one who finds him and gives him sanctuary, and though the context of his discovery the week prior is still unknown, both of them being in Quadratum is plausible. They are both "dead," for all intents and purposes.
What is known about Strelitzia is that she's possibly been the liaison between the "World of Fiction" and Sora's reality, explaining to his friends Riku and Kairi how he ended up in Quadratum. The "Nameless Star" that's depicted as a formless, ethereal voice in the Final World, helped point Riku and Kairi on to the path of Quadratum when searching for Sora. Considering Strelitzia is the only other character who's seemingly jumped between realities like Sora, it would make sense that she's been the one trying to help Sora's friends find him. She's at least shown great empathy in the past with the events of Union Cross, so this Nameless Star is likely Sora's ally.
Beginning Sora's Journey in the 'Afterworld'
The Kingdom Hearts series has always been weary about addressing "death" or otherwise perishing directly, especially with the many examples of characters finding miraculous ways to come back to life and regain their hearts. Rather than the loss of heart and the expected rise of Darkness and a Heartless/Nobody in their place, both Sora and Strelitzia seem to have suffered an even worse fate: the loss of their existence entirely. Sora and Strelitzia find themselves in the self-described "Afterworld," but it's not quite an afterlife or purgatory; rather, it's another dimension of reality that exists separate from the university of "reality" that Sora and his friends are from.
As stated in the Kingdom Hearts 4 trailer, "If this isn't the ending you desired — if it brings you despair — then leave this world for another. Your options are endless." It's a cryptic message, as are all things plot-related in Kingdom Hearts, but it's clear this is a message to Sora finding new purpose in the world of Quadratum. Strelitzia may be the key to that newfound life, and could mean that this new "The Lost Master Arc" is all about Sora's eventual return to his reality, thanks to this former Dandelion's help.
Strelitzia specifically mentions Sora's been asleep for a week before he awakens in the latest Kingdom Hearts 4 trailer, meaning she could be taking on a Namine-like role in the next game. Beyond that, it's hard to say what form that may exactly take, but Strelitzia has been taking care of Sora in this unknown reality. As for what the Master of Masters' goal or scheme is with the two of them meeting, that's also impossible to say. That's without even mentioning what role Yozora takes in Quadratum and Sora's journey, which is even more convoluted at this point. That being said, the Darkness has found its way into Quadratum, and Sora's the only one who can stop it.
Kingdom Hearts 4 is in development.