Regarding my story I posted earlier today too.
This version of King K. Rool is portrayed being as his punned name implies very cruel notably towards his minions and very easy to anger (Fighting and easily nearly killing Endeavor for setting foot on his island even when he stated he had no idea anyone lived on it or had no intentions of fighting and that he'd be on his way but King K. Rool wouldn't let him leave as to him anyone that steps foot is a trespasser who will be severely punished) and easily holds a grudge towards those (Given he created a machine just to get to other universe seeking revenge). Despite his demented nature shown to be extremely intelligent as he was able to get his minions to build a portal machine capable of traveling to other universes through the harness of the leftover energy left behind by the merge. He's also very smart, strategic and cunning in battle using trickery in battle to his advantage. Notably anything Bakugou threw at him was countered and outsmarted indicating his high IQ in spite of his insanity. In spite of his size and bulk, he's also capable of jumping great heights, speed and possesses immense strength enough to match his nemesis Donkey Kong, in spite of what his appearance may imply he claims that what those are seeing isn't fat but is in fact actually muscle. His durability also showing almost no boundaries too as he was able to tank full-fledged hits from Izuku even after powering himself up many times.
All these reasons why he ended up almost easily defeating and killing Endeavor in battle in the previous story had Izuku along with Tsuyu, Donkey Kong, and Diddy Kong not interfered in time. Along with his cruel nature, he makes sadistic yet humorous remarks, one notable case where he tells Todoroki that he nearly did him a favor in almost killing his father as he had father issues with him and that it would have been finally dealt and that he, his siblings and his mother no longer would have to deal with him.
The ending would be Crocodile Isle blowing up as Izuku, Donkey Kong and the others saves Inko and escapes. King K. Rool and his minions all survive sailing away on a gigantic raft with the Kremling King telling Izuku this won't be the last he'll see from him and that he will finally get his revenge one day with Izuku commenting on how he just won't give up and how persistent he is. Donkey Kong and Diddy Kong though tells him that they'll keep a close eye on their nemesis to make sure that he won't which Izuku thanks them for before he, Inko, Bakugou, Ochako, Todoroki, and Iida return to their own universe saying goodbye to them.
It's just so smooth.
