Hawkeye lives! Avengers 4: Endgame trailer confirms Ronin fan theory
Here are some ideas about why Clint Barton is wearing this cool costume in the movie.

Nice outfit, Clint. But what are you doing in Japan?!
The first Avengers: Endgame trailer arrived like an arrow to our hearts on Friday, and we got our first glimpse of Hawkeye (Jeremy Renner) in a cool new outfit, seemingly getting into fights in Japan and confirming a long-held fan theory.
Comic fans will recognize this as the Ronin costume, but why has Clint Barton -- who's been absent from the movies since 2016's Captain America: Civil War -- adopted this identity in the 22nd movie of the Marvel Cinematic Universe?
This costume first showed up in 2005's New Avengers No. 11.
It's possible that he lost his family -- whom we met in Avengers: Age of Ultron in 2015 -- in Thanos' deadly snap. He and wife Laura (Linda Cardellini) had three children, and losing any (or all) of them may have left him traumatized and seeking vengeance.
He was also under house arrest, as mentioned in Avengers: Infinity War, for siding with Captain America against the government in Civil War.
In the comics, Clint was the second person to don the costume, which first appeared in New Avengers No. 11 in 2005. The original Ronin was superhero/former Daredevil villain Echo.
In feudal Japan, Ronin were masterless samurai. They've captured imaginations worldwide since they were depicted as wandering bringers of justice in the 1956 classic Seven Samurai.
Clint was killed during the Avengers Disassembled storyline and resurrected during the reality warping House of M, but was (understandably) psychologically damaged by this experience. He abandoned the Hawkeye identity and rejoined the team as Ronin in New Avengers No. 27, on a mission to rescue a captive Echo in Japan.
Hopefully the Ronin subplot in Avengers: Endgame will be a little less convoluted, but it's likely that emotional trauma and/or avoiding trouble for escaping house arrest will be behind Clint's decision to take on a new identity.
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