A better way might be to explain. In Duodecim, all the characters in it on the heroes' side have limited amnesia. They're essentially being forced into a giant deific war of order vs. chaos, and have the promise of returning to their worlds with their memories restored should they be successful. Exactly how much memory they lose depends on the given hero; some, like Terra, don't remember much of anything of their own world, whereas others, like Cloud, remember most of it.
Divorced of such ties to their homeworlds, most of the characters wind up gravitating towards working together, often with unusual results. Tifa, Laguna, and Vahn wind up working together alongside Lightning, who really doesn't like Laguna's lax attitude and Vahn's complete lack of focus. She initially treats Cosmos with the same deference a soldier gives a field commander, and shows a willingness to take charge and get shit done, but she quickly learns that this situation is more than a simple war, and that her allies matter far more than she thinks they do.
Without spoiling too much, she starts to warm up to them after meeting with Cecil and a few of the others, and develops a soft spot for Firion. By the end, realizing that Chaos has developed a way to not only win the war, but to ensure that the heroes never return, Lightning performs a desperate holding action, ultimately sacrificing herself along with Kain and the others in order to ensure that everyone else survives till the next rotation of this war. The implication is that in doing so, she and the others found an alternate way to return to their worlds.
It's a really stark contrast and it gets across that her attitude and behavior is more a result of her being a driven soldier, rather than like Cloud's mental damage or Squall's compensation for not wanting to be alone.
