Forever Rachel is the name of a sad musical piece from the soundtrack of Final Fantasy VI. It is the leitmotif of Locke's fiancée, Rachel.
In the game's storyline, she was killed in an attack by the evil empire some time before the game begins, and Locke was not there for her when she died, for reasons which won't be revealed here. Thus, Forever Rachel can be thought of as her requiem, in a way. Its mournful strains fit well the tragedy of her death.