He and she were originally two parallel lines that would never intersect, but they had an intersection because of a girl.
His whim disturbed her calmness. When she decided to let him into her world, he held the girl's hand.
In order to save the girl he loved, she sacrificed herself, and what was given to her as compensation was a certificate, his promise.
However, what she wanted was never the love she begged for, so she let go, letting him find his happiness, and letting herself be free.
Four years later, they met again, but everything was different.
When the past is sealed, all they can open is the future.
Parallel lines have only one intersection point. After intersecting, they can only extend along the distance without each other until... The end.