#fushi says in literally one of their first conversations is a way of showing love. so. insanely. by lying about this they are in fact.

[bestbonnist]

Mizuha is constantly looking for proof that the people around her love her. When she asks Fushi to cry if she dies, she's essentially asking them to prove that they care about her. The question of how to prove that you love someone is asked in the case of Mizuha, and in the case of the knockers—Fushi and Yuuki spent some time searching for a way to tell the difference between humans and knockers. The solution Yuuki suggested was that knockers might not be able to feel love.

This question is also relevant to Fushi, who isn't confident that they love their friends because they're not human either. If love is a uniquely human emotion that knockers don't have, no matter how human they look on the outside, Fushi shouldn't have it either.

So Mizuha asks her question. Fushi gives her the answer she wants to hear. And she calls them out on it—not only are they lying about crying for her, they've never cried at all. To translate this exchange into the bigger picture, Fushi doesn't love Mizuha because they're not capable of love for anyone. That's what Mizuha/the left hand is saying.

Mizuha specifically asked Fushi to cry at her death. Up until this point, Fushi's watched so many of their friends die, and now Mizuha/the left hand's telling them that their grief is a lie. The most painful thing is that Fushi can't even really defend themselves because they feel like they deserve it.

Ultimately, the difference between humans and knockers doesn't matter, because Fushi decides to create a world where they can live peacefully together. During the present era, we see knockers cry and express love for others. And of course, Fushi loves their friends, even if they don't explicitly acknowledge it. So whether or not they cry has nothing to do with whether they're human or not. It's just part of who they are.