Okay, but that "ace in the hole" is a photo in his pocket that he just happens to have on him at the exact second she's about to kill him.
If his plan was "carry leverage at all times just in case," fine, but why this photo? Why is he walking around with a picture of Ao's sister like it's everyday carry? That's not being prepared. That's carrying around a very specific, very random piece of evidence for no logical reason unless he knew exactly that this confrontation would happen.
So yeah, him wanting insurance makes sense. But that specific insurance being a photo in his pocket, at that exact time, with zero setup? That's not preparation. So that just makes it contrived writing.
You're maybe right that his fearlessness is unrealistic, but the guy is being painted as some sort of sociopath the whole story. The photo being there is very specific, situational and convenient. That's the problem.