Character A and Character B are magical beings of a supernatural world. A is Evil and B is Good. Both have their battles on their world frequently and sometimes A wins, sometimes B wins, sometimes neither wins. Until one day during a battle A uses a portal to escape leaving B (but B follows later).
A is transported to Earth and realizes all magical powers are gone and is converted into a simple teenage human being. A has to learn and adopt the rules, social customs, guidelines to live as a human until A encounters B who is also a teenage human and lost all magical powers. A normal hand to hand fight begins on the street itself until the cops arrive and embarrasses them as "boyfriend-girlfriend" fighting.
They use their leftover magic to find an Identity, a job and shelter (because that magic is not enough to create the portal again).
Plot twist: They are neighbours and B (who is the 'hero') always keeps an eye on A (who is the 'villain') so that A doesn't get back the powers and use them on Earth or even return back without B. They quarrel every day on little things and A getting soft hearted on Earth pisses B mostly, which develops a rom-com and feelings unknowingly.