Mary[1], nicknamed Flee[2]is the younger sister of Jack Frost, featured in Rise of the Guardians. Jack ushering his sister to stay still, who was standing on the thin and cracked ice of the pond.
He persuades her to think of the situation as a game of hopscotch and to slowly move inch by inch. Jack saves her and instead accidentally maneuvers himself onto the same cracked surface. The ice gives away below him, and he falls to his death, drowning. It was because of this act of love that The Man in the Moon made Jack into an immortal and chose him to be a Guardian.