Santino D'Antonio is an Italian mafia boss associated with John Wick, with whom he owns a Marker. He serves as the primary antagonist of John Wick: Chapter 2, and his actions in that movie lead directly to the events of John Wick: Chapter 3 – Parabellum.
To repay the latter debt, John was bid to assassinate Gianna D'antonio, Santino's sister. He succeeded, thus making Santino head of the Neapolitan crime organization known as the Camorra.
When legendary hitman John Wick sought to leave his life as a killer and complete one last job for his boss Viggo Tarasov, John contacted Santino for help. Santino aided John in finishing what was dubbed "the Impossible Task", and in return, John made a blood oath with Santino - the Marker.
John Wick: Chapter 2
A few days after John came out of retirement to seek revenge on Viggo and his son Iosef, Santino decides to visit John Wick and presents the medallion to demand service from him. However, John refuses to honor the oath, claiming that he is retired. Santino leaves the house displeased, and opens the back of his truck, revealing a grenade launcher which he uses to destroy the house.
John later decides to honor his oath, with some convincing from Winston, and goes to see Santino to find out what his mission for him is. Santino tasks him with assassinating his sister, Gianna D'Antonio, so he could take her spot at the High Table, a council of leaders of the world's most powerful criminal organizations.
After the assassination, Santino placed a seven million dollar bounty on John to cover his tracks. However, John escaped Santino's assassins and chased him through New York, convincing The Bowery King to help rather than face a war with Santino. Santino was eventually forced to retreat to the Continental where the hotel's rules prevented killing on the property. Confronted by John in the hotel's lounge, Santino made it clear that he intended to seek refuge at the hotel indefinitely, but John shot him dead anyway. As a result, John was declared excommunicado from the Continental and the High Table placed a global fourteen million dollar bounty on his head.