Well, we don’t know for sure how far long events have progressed so it’s possible that she has settled into her role in the COG for a little while. But even so, she comes from a different background compared to JD or his father.
For Marcus and his generation, they grew up in war. Pre-Emergence society instilled a sense of duty; an expectation to contribute to both the war effort and the state. And so they served and fought just as their parents fought, and their grandparents, and great-grandparents, etc. Their reasons and feelings may vary, but it was all they knew; JD briefly made a point about this during the campaign. Speaking of whom, while JD grew up in a time of peace, he did so in the shadow of humanity’s greatest champions, with the influence of this fledgling New COG not far off, so it makes sense that he would follow in his father’s footsteps.
Kait is different, she doesn’t fully appreciate or perhaps fit in with this. She grew up in a society that is largely anti to everything the COG is. Where the COG is for order, the Outsiders favor freedom. What the COG defines as duty, the Outsiders prefer cooperation. Where the COG is rigid, the Outsiders are fluid. This is what Kait knows, and probably why she doesn’t fit the typical mold of a soldier.
I’ll admit that she did come off as moody at times, yet I don’t think there’s nothing wrong with a character acting out or letting their emotions get the better of them—in fact it makes them more interesting. But it didn’t always work out because it felt like the writers were trying to create tension between the characters that didn’t go anywhere.
There’s probably a lot that we don’t know yet as the Gears 5 trailer only provides a glimpse into the direction of the story; but considering what we know about Kait’s heritage and the implications such knowledge has for the rest of humanity, I think her behavior can be justified if given the proper context.