The background could be divided in pre-spirit era and spirit era.
Jack is who he was; there are things that don’t change even if you lose your memory. As Jackson Overland he had a family, in the flash back you can see his mother for a moment, and one of the most important people in his life: his sister. In those flashbacks we see that he is a fun-loving guy who likes to play tricks, but on the other side he loves till the end and more. When his sister risked going under the ice, he used fun to make her less scared, but in the end he was the one that ended drowning in the frozen lake. I’m hoping that in the next Guardians of Children there will be a part to show more of Jackson’s life.
Because he was selfless and gave his life for his sister, Man in the Moon noticed him, and decided that he was spirit material, and then in the future a Guardian. He took him out of the lake, and gave him the name Jack Frost. He doesn’t remember his life as human, and doesn’t know what he should be or do, but he finds out his powers and the fact is he begins having fun. There is like a really big whole to Jack’s life as spirit. The only thing that we know he did before the movie is the blizzard of ’68; for which Bunnymund is angry with him, but we don’t know why he did the blizzard, what exactly happened that Easter. Another peak in to his life is that he comments on the fact that he tried to break into North’s workshop numerous times, but the Yetis stopped him; he even says hi to Phil who makes a motion of “I’m watching you”. But mostly we know about his 300 years is that he became an expert in what he does: making snow days, having fun.
I’m really hoping for more information about Jack from the novel that
has to come out yet, and the fact that they are somehow saying that there will
be probably a sequel for the movie.