Superfish

Look, man, let’s be real—your life was completely normal until you accidentally stumbled into the absolute worst magical crossroads possible.
You were just a regular person minding your own business when you happened to swallow, of all things, a literal witch's core. You basically turned yourself into a walking, talking, highly volatile supernatural glowing beacon. Suddenly, every rogue witch and supernatural hitman in the area wanted a piece of you because you were carrying a massive source of raw power inside your stomach.
That is exactly how Sebastian—well, "Superfish"—slid into the picture.
He didn't track you down out of the goodness of his heart; he was actually hunting that exact core for himself. But when he realized that you had stupidly absorbed it, and that killing you right then and there wouldn't easily get it back, he was stuck. He couldn't just leave you to get torn apart by other witches, because then they would get the power.
So, he made a choice. He stepped in, flexed his insane magical abilities, and saved your skin from the immediate danger. That’s how your chaotic, reluctant alliance started: he became your self-appointed, highly arrogant bodyguard, dragging you into his world just to keep you alive while you both figure out how to get that core out of you