After almost two decades worth of Kingdom Hearts games, Square Enix is still dropping new mysteries for fans to unravel and speculate on as the series moves forward. One of the biggest mysteries, introduced in the Kingdom Hearts 3 DLC Re:Mind, is the character of Yozora, who appeared as one of the most difficult bosses in the series so far.
Square Enix clearly has plans to make Yozora a major aspect of future Kingdom Hearts titles, with the plot of Kingdom Hearts 4 likely revolving around the character and his battle with Sora in Re:Mind. However, with his inclusion, it is entirely possible that the developer is signaling that this mysterious new character could be the replacement for a character who has completed his arc and is ready to step out of the spotlight.
Riku, in particular, is one of many characters to complete his journey by the end of Kingdom Hearts 3, with the redemption arc from rival to villain to hero finally culminating in him literally letting go of his younger, darker self. As a result, when the series moves forward with Kingdom Hearts 4, he may serve a practical purpose in attempting to bring Sora back from wherever he is trapped, but there really isn't anywhere to take his story. This could mean that Yozora, as a dark and brooding character, might be getting primed to take Riku's place as both Sora's rival and the conflicted soul caught between both the realms of light and dark.
Riku on the Way Out
The ending of Kingdom Hearts 3 did its best to wrap up the storylines of a number of characters, like the Aqua, Terra, Ven, Roxas, Axel, and Xion, but one of the most important closures of the series belongs to Riku. By the time the credits roll, Riku has a heart to heart with a clone version of himself, going all the way back to Chain of Memories. As far as continuing the character's arc goes, he really doesn't have anywhere left to go by the time Kingdom Hearts 4 arrives, which could mean that the Riku's role might be changing from master-fighter to master-trainer, or something of the sorts.
It's clear from both Kingdom Heart 3's original ending, as well as the revelations leading to Re:Mind's secret boss fight against Yozora, that Riku will be integral to bringing Sora back from the Final World. However, considering the formula that has been developed throughout the series, it isn't likely that this section will be much more than a tutorial area, unless this is explored in a spin-off title. So, from there, Riku may not have much else to do in Kingdom Hearts 4, meaning that he may be free to take on the responsibility of training new Keyblade wielders, something the series may need in order to kickstart its new era.
Yozora as the New Rival
Looking at Yozora's introduction in Re:Mind, it's clear that he is being set up as an incredibly powerful fighter on par with heroes like Sora and Riku. Considering that his initial encounter with Sora ends in a fight, it's possible that the two will be dueling each other multiple times over the course of their continued adventures before eventually working together. This isn't too different from how Sora and Riku's relationship began in the first Kingdom Hearts, with the two fighting multiple times over the course of the game, leading all the way up to the fight against Ansem.
It's entirely possible that the same concepts will be explored in Kingdom Hearts 4, as Yozora is possibly manipulated by former Organization XIII member, Luxord, who Kingdom Hearts fan theories claim is driving the car in Re:Mind's secret cutscene. Granted, the true nature of many of these connected characters is still being explored throughout the ongoing story of the Union Cross mobile title, which is beginning to more heavily affect the current story. However, with the way that the story continues to work its way into repeating circles, recycling a similar story arc as Riku for Yozora wouldn't be too odd for the series.
Yozora as a Playable Character
The concept of a playable Yozora in Kingdom Hearts 4 isn't exactly a new theory or wish list entry among fans, but putting the character into player hands could solidify his role. However, going as far back as the first spin-off in the series, Chain of Memories, the first time player control was shifted from Sora to another character, it was Riku. Since then, players have controlled a number of other characters, but as Kingdom Hearts sets up for a new, post-Xehanort era, giving control of Yozora could set him up for a new Riku-like storyline.
As many fans immediately pointed out in his introduction as the protagonist of what most players assumed was a joke game, Verum Rex, Yozora is similar to Noctis, a Final Fantasy character that has yet to appear in Kingdom Hearts. These similarities may be superficial, but if Kingdom Hearts 4 leans in on those comparisons, then players can assume that Yozora may be a somewhat proud member of royalty haunted by doubt in his ability to take on his destined responsibility. So, while Yozora is an incredibly difficult fight when players first meet him in Re:Mind, he may end up fitting the role of an uncertain rising star while he is controllable.
Riku Trains Yozora?
If the plan really is for Riku to pass the torch to Yozora in the next Kingdom Hearts title, then the best plan for both characters moving forward could be for the old favorite to take his role of Master as a teacher. With both characters appearing to be around the same age, this may seem like an odd choice, but it's clear that Yozora isn't a Keyblade wielder yet and will need training to earn the Mark of Mastery. Even though Yen Sid or Mickey might then sound like better choices because of their seniority, the connections between Yozora and Riku might make them a better pair to work off of each other.
The similarities between both characters has even been commented on in-universe, with Donald and Goofy comparing the two when they first see Yozora's picture on Verum Rex. Setting up these two together in a Mentor and Student relationship could be a quality payoff for this initial introduction, making up for some of Kingdom Hearts 3's more disappointing aspects. With one character stepping down and the other stepping up, this may be the best way to ease fans into this new era for Kingdom Hearts.
Kingdom Hearts 4 is not confirmed to be in development.