Leonid Arkadin is Jason Bourne's equal and archnemesis who appears in The Bourne Sanction, The Bourne Deception and The Bourne Objective. He's almost as skilled, as damaged and as brilliant as as Bourne.
The Bourne Sanction
In The Bourne Sanction, Leonid meets a girl named Devra, whom he falls for. Throughout the book, he engages in a cat-and-mouse chase with Jason Bourne, during which Devra is killed by Semion Icoupov. In a bitter rage, Arkadin pursues Icoupov and Asher Server, and later kills them both. In final battle in Moon of Hormuz, Arkadin decided to help Bourne before fighting against him. In the end, Bourne sent him overboard from the ship and Arkadin fell down into the sea.
After that, Willard, Conklin's mole in the NSA, revealed that Conkiln wanted to create the ultimate warrior in Treadstone. It's pity that Conklin--killed by Stepan Spalko in The Bourne Legacy-- was not alive to see who would triumph: Bourne or Arkadin.
The victor was Bourne - the world's ultimate warrior.
The Bourne Deception
In The Bourne Deception, Arkadin develops a rather complex plan to trap Bourne. If Bourne kills Nikolai Yevsen, Arkadin takes over Yevsen's business, and if Bourne is killed by Yevsen, then Arkadin's vengeance on Bourne is satisfied. He wins either way. In the book, much of Arkadin's past is revealed during Arkadin's adventures trying to take over Nikolai Yevsen's business. Arkadin greviously wounds Bourne, who fakes his death in order to find the answers behind his murder attempt. Eventually, they confront each other in Syria, during the Black River Scandal. Arkadin shoots Bourne three times in the chest, but Bourne survives and later escapes, much to Arkadin's surprise.
The Bourne Objective
In The Bourne Objective , Arkadin continues hunting down Bourne. At one point in the novel, he discovers that Moira Trevor is a friend of Bourne and consequently wounds her with a conch shell. At the climax of the novel, Arkadin meets with Bourne, who tells him that the terrorist group Severus Domna has manipulated the two of them in an attempt to get them to kill each other and that the only way to surivive is to work together, thus surprising them. Arkadin is reluctant, but eventually agrees. During the mission to unlock the secret to Solomon's Gold, things go awry when it is revealed that the laptop that unlocks the secret to King Solomon's Gold is fake. Arkadin, not believing what he hears, lashes out against Bourne and then goes in a frenzy, attempting to unlock the fake laptop with King Solomon's Ring, despite the room they are in being rigged with explosives. Consequently, Arkadin is killed in the explosion, though Bourne manages to escape with his life.
Arkadin has a tragic past, the victim of child abuse courtesy of his mother, who locked him in a basement. As a result, rats chewed off two of his toes. Arkadin's past tragically haunts him, leading him to envy Jason Bourne for his memory loss. Following Devra's death, Arkadin became consumed with revenge, and eventually became hell-bent on killing Bourne.