He's Angra Mainyu, All the World's Evils, the corrupting mud inside of the Greater Grail, and the main antagonist of that one, and on top of that he's taking the form of Shirou Emiya who Karen is familiar with. He's aiding Chaldea because without the Greater Grail he's actually pretty harmless (he's the only 0-Star Servant and honestly he's kinda usless) and he doesn't actually hate Humanity.