Jack Landors and Elizabeth Delgado have been best friends for years. In fact, their relationship has been described as "brother and sister," and the strength of their bond has proven true time after time. Jack is always reliable when Z needs backup, trusting her enough to follow her lead even to the point of joining SPD. And Z's affection for Jack is obvious; when she's faced with a dilemma, it's Jack to whom she turns. I think these two share a bond of deep friendship born from mutual dependency and hardship, but it's not a quantum leap to imagine their friendship transforming into something different.