Who is the character John Wick's former rival, Cassian, and how does he fit into the John Wick storyline?

Cassian is a highly skilled assassin who used to work for the Camorra crime syndicate. He is first introduced in John Wick: Chapter 2, where he serves as the bodyguard of Santino D'Antonio, an Italian crime lord. After John Wick is tasked with assassinating D'Antonio by a marker, a blood oath that compels him to do the job, he reluctantly takes on the job and kills D'Antonio in the Continental hotel, which is supposed to be a neutral zone for assassins.

Following D'Antonio's death, Cassian seeks revenge against John Wick for killing his former employer, even though it goes against the rules of the criminal underworld. He engages in a brutal fight with John Wick in the streets and even manages to injure him, but eventually comes to a truce with John, after realizing the gravity of the situation.

Cassian is played by the actor Common, and his character adds an interesting dynamic to the John Wick storyline. He serves as a reminder that even in the criminal underworld, there are rules that need to be followed, and consequences for those who break them. His character also showcases the loyalty and honor that can exist among criminals, even if they are on opposite sides of a conflict.