Wick, prior to being a hitman, served in the United States Marine Corps. This can be noticed by the tattoo on his back, which from Latin to English says "Fortune Favors the Brave", the motto of the United States Marine Corps, 3rd battalion, 3rd Marines.
Prior to PAYDAY 2
John Wick went dark after love made him retire from the career of a hitman, helping the Russian Mafia rise to power in exchange for a quiet exit from the criminal life to marry Helen, the love of his life. But a few years after he went legit, Helen Wick contracted terminal cancer. When Helen passed away, she left John a posthumous gift of a dog named Daisy as a way to help him grieve. When the son of Wick's former boss took away the only link left to the life that he left behind during a reckless car theft, Wick in turn eliminated every last member of the criminal syndicate, including the man who granted him his freedom. Since then he has appeared in Washington to continue his criminal trade and to remind D.C. of one of the most feared hitmen in the criminal underworld.
According to Wick's Crimefest description, he and Chains were former comrades during their
time in the military. Apparently Wick has just resurfaced after a time of "going dark" to lend Chains (and his buddies) a hand.
Based on in-game interactions, Wick and Hoxton have apparently known each other for quite some time, or at least have heard of each other, though the former case is more probable due to Hoxton knowing about Wick's family.
As seen on the Crimefest site, Wick apparently has an affinity for shades/sunglasses, as he wears a pair as his signature "mask", and a couple new sunglasses were introduced alongside him.
The FBI Files
A former hitman previously associated with New York Russian mafia boss, Viggo Tarasov. Suspected of being involved in the mass executions of Russian mobsters on the East Coast. Understood to have gone dark after the massacres at the Red Circle nightclub, but drawn back into the criminal life for unknown reasons.
Wick has proven to be a highly effective weapons expert. His presence within the Payday crew is perplexing to our analysts, however. Wick has a fearsome reputation as a hitman. Therefore, why is he participating in armed robberies. Though undoubtedly offering unique skills to Crime.Net, the questions persists - why does Bain need an assassin?