What Once Was Mine

The ice broke.
I heard a terrible sound, like a thousand mirrors shattering. The world whirled around me, shooting upwards - or was it me that was falling? Something hit me in the side, and I felt a crack, like the ice, but this one was inside me. The lake surface was pulled out from under me like a rug and I flew sideways, blurring again.
Pain thundered through my head.
Everything went black.
-
Three hundred years ago, a boy gave up his life to save the girl he loved.
That night, he woke.
With one memory, one name;
Elsa.
Alone for three centuries, he finally finds his place among the guardians, and helps them defeat the villainous Pitch Black once and for all.
But afterwards, all is peaceful. And he is bored.
So when Manny picks out a new guardian, Jack Frost is assigned the task of showing her around.
But what if she is someone
That once was his?