in the time that is given to us
There is very little that she can do, sometimes.
He has all the weight of power and position and age on his side, and she cannot physically stop him walking into danger because he thinks that he should protect her or the rest of the Division, or that it is a job for him to handle, or any of half a dozen other reasons that make no sense. He is stupidly reckless. He gets away with it. One of these days he is going to get himself seriously hurt.
Law of averages.
These things happen.
There's always someone bigger and more dangerous and more powerful and just because he's survived a couple of thousand years already doesn't mean that he's immortal or invulnerable or Yamamoto-soutaichou himself.
(Everyone knows that Yamamoto-soutaichou's not going to bother dying until he's got things 'properly organised', which means never.)
He protects her when she doesn't need protecting and puts himself in danger because of it, and when she tries to protest, he simply smiles at her and tells her that a pretty young girl shouldn't have to worry about that sort of thing.
The worst thing, really, is that she knows why.
There was a previous vice-captain, whom Nanao remembers very clearly. (There always are previous vice-captains, but some leave under good circumstances, and others . . . don't.) There was an accident. Or a Hollow. Or a death. Definitely a death.
The details are sealed.
And even in the moments when he is most flown with wine, most talkative and most vigorously expansive, that is the one thing that her Captain won't talk about.
She could resent the other woman for getting herself killed, but that would be petty and unproductive, and against all the memories that Nanao still has of her, those memories of respect and admiration and even love. But it's a weight which Nanao has to carry now, an expectation that she has to endure, and a need to watch her Captain's back because he is too preoccupied with trying to watch over her.
The only comfort that she can give him is her own strength.
Maybe some day she'll be able to heal the guilt that her predecessor (observe the root verb, pre-decease) left behind her; maybe that'll be enough to take a little of the shadow away from his eyes. Maybe. She can wait for the cuckoo to sing. She's a patient woman.
The thought comforts her, a little, in those moments when there is very little that she can do.