King K. Rool was the villain of the Donkey Kong Country series for many years, but his significance was downplayed following Microsoft's acquisition of Rare, which meant that King K. Rool was phased out of the later Donkey Kong games. The fans never forgot the royal lizard and King K. Rool was one of the most popular requests for the Super Smash Bros. series. It took until Super Smash Bros. Ultimate for Masahiro Sakurai to grant the fans' wishes and add King K. Rool to the roster.
A strange scene occurred during the reveal trailer for Banjo-Kazooie in Super Smash Bros. Ultimate, which was shown as part of Nintendo's E3 2019 Direct. The trailer opens with King K. Rool sleeping in Donkey Kong's house, while Diddy Kong & Donkey Kong are hanging out. It seems strange that the Kongs would be chilling with their main villain, in the same way that it would be strange if Mario and Princess Peach let Bowser sleep on their couch after a tough day of go-karting.
It's possible that there could be more to the new-found friendship between King K. Rool and the Kongs that anyone realized. A Twitter-user named @Ethan_Mouta has uploaded a theory that links the appearance of Joker from Persona 5 to the change in King K. Rool.
I think I may be the first person who realized this! pic.twitter.com/mQGQPNAzjc— Ethan Mouta | CrashForSmash (@Ethan_Mouta) June 13, 2019
In Super Smash Bros. 3DS/Wii U, there is a trophy for King K. Rool with text that states that he could be Donkey Kong's most trustworthy friend, but that it was unlikely that he would have a change of heart. In Joker's official character trailer for Super Smash Bros. Ultimate, there is a scene where he is shown stealing King K. Rool's crown and running off with it, with King K. Rool in pursuit.
In Persona 5, Joker and the Phantom Thieves had the ability to enter the minds of corrupt individuals and could steal a treasure from within their consciousness. If the treasure was removed, then that person would be forced to see the error of their ways. It's possible that the reference to a change of heart in the last Super Smash Bros. game was an intentional hint at what was to come, with Joker stealing King K. Rool's treasure (his crown) and causing him to change his ways and befriend the Kongs.
Banjo-Kazooie are coming to Super Smash Bros. Ultimate in autumn 2019.