Story Hero Overwatch: McCree, Snipers from the American West!
As a young man, McCree is a member of Deadlock Gang, known for selling illegal firearms and military devices throughout the Southwest of America.
McCree and his colleagues were caught in an operation carried out by Overwatch. Seeing McCree's shooting and cleverness, the head of Blackwatch's secret division, Gabriel Reyes (before becoming Reaper) visited him and made an offer.

In the days of King's Row Uprising, or the Omnic rebellion perpetrated by the Null Sector, Blackwatch and its agents were in the process of investigating on treason charges, resulting in the division becoming a vacuum.
At the time of the incident of King's Row Uprising, McCree, Reyes, and Jack Morrison (Soldier 76) were assigned in the city of London. In fact, officially under the orders of the prime minister, no Overwatch agents are allowed into the UK territory!
Although McCree's presence had initially been deemed to have violated protocol by Ana Amari and Jack Morrison, but at the time of the Null Sector uprising, McCree was the one to report on the hostage situation to Morrison and Reyes.

As you can see in one of these comic scenes, it appears that McCree has been wearing a Blackwatch uniform that is currently the skin of McCree's hero in the game.
When Overwatch begins to lose its influence and engages an internal conflict with Blackwatch's betrayal, McCree decides to leave Blackwatch and disappears.
He reappears a few years later as a sniper. With his extraordinary talent, many organizations have actually wanted to recruit McCree, but he likes to move on his own and fight for what he believes!
Image: McCree