I understand what you mean.
Let's talk about Jamie first. He's obviously supposed to represent Jack's younger sister in a way, believe it or not (because these movies always holds symbolism) and he has the similar rowdiness and energy. Also, brown and and brown eyes really tells the character off. He's an example of a full-out believer--- an enormous amount of wonder, hopes, and dreams fill up inside of him, and his memories revolve around Jack Frost who makes his days more fun. These five things represent the Guardians, and Jamie is in a sense their symbol of a perfect childhood. He's respectful and he admires his elders very much, and also is Jack Frost's future of continuing his older brother figure.
Jack Frost may be known as a mischevious immature boy, but who knows? 300 years living alone with no one to talk to can really change someone, being a spirit or not. Many things were thrust at him after decades of isolation: his memories, being appointed as a Guardian, and so and such. You can tell that he's a responsible being by the end of the movie, and he's sorted out his priorities that protecting children and their rights is something necessary to do. It's kind of branching out from him protecting his sister before he died. Jack loved his sister very much, and if he'd die for her as well, then PHEW, is he a good brother. This love is passed down to the future generation of children, and Jamie seems to be the more broad vision of being protected by the Guardian of Fun. And dear, if you're writing a romance fanfiction, please don't make it cliche like the "Jack runs into a girl that somehow sees him and they fall in love" because 1. He seems to be the type where he likes "one night stands" and flirting instead of going on something serious and 2. He's still deducted as a BOY, not a BACHELOR. I want to see character developments and whatnot. Sorry if you aren't writing about romance, but I really wanted to get that out.
Sorry I replied so late!