What is the backstory of the character Santino D'Antonio, and how does he fit into the John Wick storyline?

Santino D'Antonio is the primary antagonist in John Wick: Chapter 2, portrayed by Italian actor Riccardo Scamarcio. The character is a member of the Camorra crime syndicate, and the head of the D'Antonio family. He is also a former acquaintance of John Wick, and calls on him to repay a blood oath that Wick had made to him many years prior.

The backstory of Santino D'Antonio is revealed throughout the film. It is revealed that he was once a close friend of John Wick, and that they shared a mutual respect for each other. However, when John wanted to retire from the life of an assassin, Santino D'Antonio made him an offer he couldn't refuse - to complete one final job in exchange for his freedom.

After John completed the job, Santino D'Antonio ordered a hit on him, resulting in the death of John's wife Helen. The film's events take place years later, when Santino D'Antonio calls in the blood oath, asking John to assassinate his sister Gianna in order to take control of the Camorra family.

Santino D'Antonio is a key figure in the John Wick storyline, as he is the one responsible for drawing John Wick back into the world of assassins, leading to the events of the second and third films in the series. His character is also responsible for the death of John's wife, making him a particularly personal antagonist for John.