Constantine and Margaret met when she was a rookie officer in the London police and the two had sex in a tavern bathroom. They started a relationship, but Constantine quickly ended it, claiming he didn't want her to get hurt in his magical adventures. However, he would later admit it was actually because he didn't like how good she was.
Marget Ames was created by Tim Seeley for his run on The Hellblazer.
Major Story Arc
For more information see: The Hellblazer: The Inspiration Game
After Constantine had been banished and returned to London, he ran into Ames, now a Detective Chief Inspector, when Constantine was tricked into murdering Doug Rindhurst by two chaotic dwarfs. As Ames investigated the murder, she, too, was made a target of the dwarfs, who kidnapped her and challenged Constantine to get her back. After bending reality with the dwarfs' enchanted liquor and killing them, the world was returned to normal, and Margaret sought to re-spark her and Constantine's relationship, only to have Constantine shoot her down.
Good Old Days
For more information see: The Hellblazer: The Good Old Days
A couple months later, after a few more adventures, Constantine returned to London in hopes of shagging Margaret, having found himself incapable of "performing" with anybody else since their last encounter. However, when he arrived at Ames' apartment, he had found that it had been turned into a portal to Hell. Ames, who was magically vulnerable due to her last encounter with John, had been used to resurrect the dead brother of Father Adam Day, who himself had caused a bridge to Hell to be opened.