The events of the Reunion with the Black Organization case[2] open the possibility that the reason they know each other so well is because Gin and Shiho had a romantic mutual relationship. In Haibara's dream, Gin describes their meeting as a reunion: "This is the end of your wanton sporting... Now, wake up from your dream... Shouldn't we celebrate our reunion with your favorite color scarlet... right, Sherry?"[3] Haibara uncharacteristically yells "Don't touch me!"[4] when Ayumi places her hand on Haibara's arm, snapping her out remembering her dream. Later, the real life Gin thinks of her naked once, looking forward to their meeting with one another. Cases later, in the Mystery Train case, he does so again.[5]
Furthermore, when the two of them meet on the rooftop of the Haido City Hotel, there are more suggestive exchanges. Gin purposefully chose not to do his job in the most efficient fashion -shooting Shiho in the chimney- but instead chose to make a special exception for her by waiting out for her on the snowy rooftop because killing her there would be, paraphrasing, more beautiful.[6] She even gets into the act by thanking him, albeit sarcastically, for "waiting for her in the cold"[7]; Shiho seems to purposefully be alluding to the stereotypic scenario of one lover waiting for another in poor climate. Gin also makes a comment about her disguise, the uniform and the glasses, being unsuitable for her, which carries an implication that he thinks there is some other outfit which would be more suitable