He and she are the opposite.
But her upper body is his lower body.
If the upper body is him, the lower body is her.
When they cross the sea, the two become humans with the same tail.
Who are they?
Note: This riddle is from Japan, specifically, a book in the S&M series
Hint:
If you cross the sea from Japan, you arrive in US - and US uses English.
What would it take to make the word ‘run’ or ‘swim’ a person?
Come to think now, 2nd and 3rd clues are almost impossible to solve for an English native… I��ll give an explanation if someone gets the right answer!