Look, I agree with everyone that Kairi's treatment was horrendous, but I've been made aware (I didn't pick up on it either when I played) that there is actually a legit reason that she doesn't even struggle: She simply
can't.
Xemnas twisted her arm behind her back. He doesn't just hold her arm up, he pulled her wrist backwards and up or else we'd see her armpit instead of her shoulder:
And if that wasn't enough, he pulls on her arm twice, clearly making her wince in pain. Try twisting your arm into that position, it hurts at a near 90° angle already and Xemnas clearly pushes 100-120° here. If she struggled, he would have broken her arm or dislocated her shoulder and then she'd definitely be useless.
That doesn't by any means excuse how Nomura wrote her, I'm just as infuriated about it as everyone else, but it's unfair to keep bashing her as a character when this one (and technically everything else since Nomura is the one who is doing the bullshit to her) is not her fault. At all. You can't forget that Xemnas is Terra's Nobody and we saw how strong Terranort was just a few cutscenes earlier.