I actually quite like what the author does with Kunon and Cassis in the novel. In the novel he acts that way because he thinks Cassis is a guy but then he learns that Cassis actually has the soul of a woman and so he treats her much better and acknowledges Cassis as being a girl while apologizing for what he did. It’s a really nice moment from the author where they have Kunon accept an transwoman for who she is rather than what Kunon thinks she is and is being trans affirming towards her
Have you read the novel? Cause I’m going based off the novel in my comment and said novel makes it clear she is a transgirl. She gets upset/mad because he treated her like she was a guy when she is a girl