The Super Smash Bros. series is one of Nintendo's most popular. It's seen a release on every one of Nintendo's console since the N64, bringing together Nintendo characters from all over to form a unique fighting game.
As the years went on and the roster of playable characters continued to grow, third-party characters not owned by Nintendo eventually made guest appearances. Snake from Metal Gear Solid and Sonic from Sonic the Hedgehog were the first to kick things off in Super Smash Bros. Brawl for the Wii.
Additionally, the majority of DLC for Super Smash Bros. Ultimate has added more third party characters like Joker from Persona 5 and Sephiroth from Final Fantasy. One often hoped for character that fans have yet to see join the roster is Kingdom Hearts' Sora. Now thanks to modder Mastaklo, Sora joins the roster unofficially.
Sora's appearance in the mod is based off his appearance in the recent Kingdom Hearts 3, and he comes equipped with the default Kingdom Key Keyblade. Unfortunately, as Sora is just a mod, he doesn't have his own unique moveset. Instead he replaces Shulk, which means he shares all of the same moves that Shulk does.
Any players who main Shulk and love Sora could find a perfect combination here. While Sora does share Shulk's moveset, he does not share his voice lines. Mastaklo has added Sora's own lines from Kingdom Hearts into the game. As shown in the example video provided, there are unique voice lines even when Sora uses Shulk's Monado Arts. Sora also has a unique line given at the end of the match, should he win. The announcer will still declare that Shulk has won however, but that was likely unavoidable given that there would be no real way to replicate the announcer.
Admittedly, it also looks a little odd to see the Keyblade adopting the characteristics of the Monado. One can hope however, that Nintendo will eventually add Sora into the game officially as DLC. It wouldn't be entirely out of the question given only three of the characters added after Super Smash Bros. Ultimate's launch have been Nintendo characters, those being Byleth, Piranha Plant, and Min Min. Sephiroth being the most recent DLC character may have hurt Sora's chances, though, as the game now has both Sephiroth and Cloud from Final Fantasy. While Kingdom Hearts is of course an entirely different series, it does borrow a lot from Final Fantasy (including many of its characters), so Nintendo may conflate them as effectively belonging to the same series and want to limit its representation.
Super Smash Bros. Ultimate is currently available exclusively for Nintendo Switch.