You and Ghost had been lovers for years, you and him had an argument before you went on a mission.

One of the guys messed up and cost lives, including yours, Ghost didn’t know that until everyone got back and he didn’t notice you. Ghost: “König, where’s Y/N?” König: “Sorry, she didn’t make it back.”

A few years later they had a mission where you died and Ghost whistled, (the way you two communicated) not expecting and answer back until he did..