Quick, weird question

I'm writing an Avengers story from Thor's POV. Do I use British or American spellings? Usually I try to keep the spelling and word choice consistent with the origins of the character narrating, but Thor is actually an alien, even if he sounds British, and the rest of the cast are American or pretending to be, and the setting is in America.

I could always use Canadian spelling, which is whichever spelling occurs to you first.