"What does she put her life on the line repeatedly?" - It's kind of her Job. She has good armour, a good shield and she's a soldier by trade.
"Does she feel guilty about hiding from danger at some point in her past, which led to the death of a mentor/loved one?" - Not really. She does however wonder why she even became a Soldier in the first place though as the allies she fights by have far greater 'power' than her. I guess you could say that she isn't quite clear on why she is valuable.
"was she small and weak growing up and feels she has something to prove?" - Not really though the people in her home country often felt disapointed after meeting her as they expected more from 'The Human of the Orc Army'.
"Does she feel, for some reason, beholden to the others, or like her life is worth less than theirs?" - Perhaps. She believes her reason for fighting is because she is getting paid to work but she doesn't understand why she is in the position she is in. Afterall, she's not some sort of battlemaster. She's just a fighter amonst wizards, warlocks and clerics.
"Is she just sworn to defend anyone weaker than herself? If so, why? And to whom did she make the oath?" - She is committed to doing her best . . . she's often the first to step into battle or to try and save people. As for some sort of Oath . . She sees being a soldier and her duty as a way of life. She isn't exactly part of an army right now either so she's like very sturdy fodder for a bunch of people she doesn't know . . .
By your responses to comments in this thread, I get the impression that she is a very well-conceptualized character. She's like a staple food, nourishing but nothing to get your taste buds all a-tingle, and you're looking for some spice.
She is stolid and dependable, a salt-of-the-earth type, which we all depend on, whether in a game or the real world.
She also sounds lonely. Her time in the orc army surrounded her with like-minded comrades, united in a focus on gaining success in battle.
Now she's amidst allies who apparently take her for granted, and (through your agency) she's experiencing something of an existential angst. Her new comrades have different world-views, different confirmation biases. She has become a minority, adrift in a mainstream culture (to the extent that the party is the whole of the culture).
Lyrics from the late singer Peggy Lee's hit song come to mind:
"Is that all there is?
Is that all there is?
If that's all there is, my friend,
Then let's keep dancing.
Let's bring out the booze
And have a ball."
In Human's situation, one could substitute the word "fighting" for "dancing".
Her natural reticence keeps her from acting out, practiced at tamping down her own feelings and deflecting any inner resentments.
I think she needs a hobby.
If you're just looking for something to give Human more "living color", maybe she could add some new pastime to her repertoire, something outside of her usual activities. Bird-watching? Collecting some class of items?
Maybe, in her downtime between adventures, she could become an instructor for a local militia. That would be more in keeping with her lifetime of focus, but ... let's start from there and give it a twist ...
All that melee training has given her a lot of experience with the importance of footwork (especially with her enduring leg injury).
Can you see her parlaying that into an appreciation for dancing? Depending on the culture, it would likely be in the line of folk dance, and might be limited to festivals and/or celebrations. It would keep her physically active, and provide that lift to the spirits that exercise, even moderate, is so good for. Her stubbornness to overcome the problems her bum leg presents could be a part of it.
Seems I've come back around to Peggy Lee. Sheeesh!
