Killer Keemstar (キラ・キイムスタル) is the antagonist of Scary Godmother Part 6: Stone Ocean. Before the events of Part 3, Keemstar had personally known Jimmy Brando before he fucking died. Because if this, Keemstar had it out for Hannah and decided to get his revenge with the power of his stand, News of the World.
Keemstar's stand, News of the World, turns anything it touches into news.
News of the World has multiple sub-abilities, one of which is I'm Fast As Fuck, Boy, allowing Keemstar to run fast as fuck. During the end of Stone Ocean, Keemstar uses this ability to run so fast that he resets the universe and somehow kills Hannah and Scary God Mother in the process.
Keemstar, being the anime villain he is, has multiple forms.
Keemstar's Human Form also happens to be his default form. In this form, Keemstar relies mostly on his stand. Rather than his own physical abilities.
Keemstar's true form. When Keemstar manages to go really fast using I'm Fast As Fuck, Boy, but not quite fast to reset the universe, he transforms into his gnome form. His strength is enhanced, however Keemstar still relies on his stand to fight. During his Gnome Form, Keemstar has a new stand, Don't Fear The Reaper, which fucking kills everybody through some means that I can't come up with.
Trough dues-ex machima means, Keemstar uses the power of News of the World, I'm Fast As Fuck Boy, and Don't Fear The Reaper to make him transcend human understanding and making him the most powerful being to ever exist. However, because he has transcended beyond human understanding, he cannot do anything to his enemies, meaning Keemstar loses by default. When Keemstar transcended in his fight against Scarce during the end of Part 6 after resetting the universe 20 times, Scarce was unable to comprehend Keemstar's new existence, meaning Keemstar could not win, so he lost the fight.