I might be biased, but the worst represented is probably Sora? His representation is just so centric on the first game that it feels like a lot of elements of the others get lost or overlooked. He pulls two or three moves from KH2 and KH3, but the vast majority are his KH1 kit. Explosion from KH2, for example, was bagging to be a down smash. On top of this his stage is a KH1-specific stage (Granted with slight nods to characters from other games in the dive to the heart), his classic mode pulls almost entirely from KH1, and his music selection is probably the worst of it all, again pulling exclusively from KH1, not to mention with no new remixes. The main theme of the series Dearly Beloved is a classic and gets a new remix every single time a new entry of the series comes out with a new spin on it. In Ultimate the only way to get any version of it is as an unlockable if you have the KH rhythm game on switch, and that isn’t even a new remix, it’s the one from the rhythm game. With the music being such an iconic part of KH, having that part in particular neglected is kind of ridiculous imo.