Christabella was conceived by Satan before Satan took her under his wing at some unknown point in her life. Satan - under the guise of John Milton - then established a law firm and gave her a position that would keep her in plain sight of a certain someone.
When Kevin Lomax arrives in New York to join said law firm, she catches his eye. Kevin is happily married at this point, so he's able to resist her charms to an extent. While at a cocktail party, Milton managed to separate Kevin and his wife Mary Ann so that Christabella could talk to him. Milton then convinced Mary Ann to cut and dye her hair so that Kevin would find her less attractive. Milton then allowed Kevin to work extra hours of his own free will so that some distance would be created between the happy couple. He even encouraged Kevin to spend more time with his wife, confident that Kevin's obsessive personality would prevent him from doing so. All the while, Christabella was in his line of vision.
After Mary Ann has a nervous breakdown and kills herself, Kevin confronts Milton, knowing now that he's both his biological father as well as the Devil. Milton then reveals to Kevin that Christabella is his half sister and outlines his master plan: to have his half-human children copulate and produce a child thats simultaneously human and devil (a.k.a. The AntiChrist). Kevin seemingly submits and lays with Christabella on an altar, admitting that he's wanted her from the moment her first saw her. At the last moment however, Kevin pulls out a gun and shoots himself in the head. Enraged with the reality that his plans have come to nothing, Satan kills Christabella.