Clint Barton. The thing that confused many ones was his "Yo!", in current storyarc. And i was skeptical too. But I just reread the Chuck Austen storyarc with the introduction of Lionheart. in one of the issues, Hawkeye says "Yo", exactly the same way.
This could mean nothing, but here's a good clue, I guess...
> Clint Barton.
...plus, doesnt Clint Barton know how the avengers banter about during a fight? This ronin character seems to be new to group dynamics, while hawkeye has been an avenger since, like, forever.