The one person (or thing) in your life who gives you an endless supply of reasons to hate them but who you inexplicably cannot imagine life without. An even more vehement title than frenemy. Could be used to describe the relationship between Magneto and Professor X.
Hard-boiled, lonely detective: "Damn you, liquor. You've ruined my marriage and chased off everyone close to me, but you make life worth living. I love you, my fremesis."