What is the backstory of John Wick's mentor, Marcus?

Marcus is a character in the John Wick franchise, portrayed by actor Willem Dafoe. He is a former hitman and mentor to John Wick, the protagonist of the series. While Marcus' backstory is not explored in great detail in the films, there are some hints and clues as to his past.

In the first film, it is revealed that Marcus and John have a long history together, and that Marcus was instrumental in helping John become the feared assassin he is. Marcus is shown to be a highly skilled marksman and is respected in the criminal underworld.

It is also hinted that Marcus may have retired from the hitman life before John, as he is shown to be living in a luxurious apartment and enjoying a relatively peaceful life. However, he is drawn back into the world of assassination when he is hired to kill John by a rival hitman.

Overall, while Marcus' backstory is not fully explored, it is clear that he and John have a deep bond and that Marcus played an important role in shaping John into the skilled killer he is today.