HumanitySerah is not a soldier.
Neither is she a hunter.
On Cocoon, the people were nothing more than fancy lab rats, catered for by the fal'Cie until they deemed it appropriate to move forward with their plans of forcing open Etro's Gate. Even when she and the survivors of Bodhum leave to make a new home, Serah provides in a different way, as a teacher. Even then, she's never had to fight, probably not even in an emergency. To begin, Serah's not a protector, not someone who defies a government, not someone who crosses the timeline and makes changes. When Serah first becomes a l'Cie, she's a victim, a casualty.
She's just a civilian. And she never stops being a civilian after returning from crystal stasis.
This also makes her extremely human.
To everyone juxtaposed against Serah, who she means the most to - Lightning, Snow, Noel - Serah is a stark contrast. Because she has never had to fight, and has had the chance to live a normal, loved life, she has a keen morality, and a good sense of right and wrong. That's to say, she knows what is fair, and what an individual deserves and needs to be happy. It doesn't mean she hasn't been through hardship before, having lost her parents, her sister, becoming a l'Cie - being a l'Cie meant that Serah saw some of the worst qualities humanity had to offer, but she held fast to her morals. Serah still believes the best in people, and that keeps her going. But Serah has never had to live in an extreme for her entire life. Noel, for instance, knows death well. That's the way of things, given that he's the last living human. But Serah has known otherwise; she knows life can be good and full, and when it's torn away prematurely and she hasn't naturalised to it, it hurts her.
Serah struggles throughout time, because everything affects and changes her. She is not a cold person, and even displaced by time, out of herself and Noel, she is the one that connects with people as well as events. This involvement can be just as much a bad thing as a good one, because as a result, Serah deeply feels everything that happens to her up and down the timeline. To Serah, people are not the numbers they would be to a soldier. Watching the people of Academia become cie'th and losing Yeul soon after almost makes Serah crumble.
Serah can't rationalise those loses as right, or justified. Neither does she ever really forgive herself for causing the Purge. There's no getting around her involvement. Granted, Serah was a plaything of the fal'Cie, but had she not indulged in her curiosity, a lot of people wouldn't have died. The inhabitants of New Bodhum have clearly long since forgiven her, or else she would not live with them, but when Alyssa talks about suffocating to death in the raid, it reopens that old wound and it's clear that Serah has never forgiven herself. She can't ever forget what she did. She won't. A casualty is not a number. She has to live with her choices. That's what keeps her human.
Coupled with her femininity, humanity is also a rare trait for the protagonist to have, and in Serah's instance, to keep. She is tested relentlessly, brutally, over and over. It's one of Serah's greatest stepping stones, the thing that makes her falter, and sometimes, she can't keep going. Serah is a champion for humanity, and so she remains human. She remains that same civilian who is sometimes overwhelmed. Serah cries, Serah falls: but she picks herself up and forces herself ever onward.
And she is made all the more relatable for it.