What Series He/She is from: Doctor Who
How could He/She mix into a Pokemon Universe (In other words. How did this person get here and why):He is timelord (a last one of a civilization that was the first created by arceus ,he also has a very strong bond with arceus.)that stuck in omni region, and you are the first person that saw him. He sense a dark power that brought him and TARDIS to this world.
What Pokemon should He/She have:
He should have some pysihic pokemon like elgeyem or beeheyem,Or prethistorical pokemon. Maybe some legendary that controls time and space and distorsion world.
That you guys should pick. The best to balance game.But i think that this character would be awesome.