[Well, it's not the first time and it won't be the last time that his general opinions in (or about) battle have been succinctly ignored...
More than that, Hayame is stubborn, as he's seen before. The kind of stubbornness that can get a person killed. Matsui truly doesn't have anything to fight for here, nothing at stake that he'd hate to lose. Getting Hayame out safely is his primary goal, and if that means letting her try and either succeed or fail to the point that retreat (hopefully) becomes the obvious answer - then that's what he'll do.]
...Then I will have your back.
[Not that he knows exactly how much he can do when he has to lose Both Hands every time the Siren starts her song... But it's still a little more than what Hayame's working with. They're running low on seconds, but as a sword, it's in his nature to ask:]
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More than that, Hayame is stubborn, as he's seen before. The kind of stubbornness that can get a person killed. Matsui truly doesn't have anything to fight for here, nothing at stake that he'd hate to lose. Getting Hayame out safely is his primary goal, and if that means letting her try and either succeed or fail to the point that retreat (hopefully) becomes the obvious answer - then that's what he'll do.]
...Then I will have your back.
[Not that he knows exactly how much he can do when he has to lose Both Hands every time the Siren starts her song... But it's still a little more than what Hayame's working with. They're running low on seconds, but as a sword, it's in his nature to ask:]
What is it that you wish to do?
[He crafted to follow, after all.]