Comic fans will recognize this as the Ronin costume, but why has Clint Barton -- who's been absent from the movies since 2016's-- adopted this identity in of the Marvel Cinematic Universe?
It's possible that he lost his family -- whom we met in in 2015 -- in Thanos' . 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, 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 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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