Which of my characters has been the most difficult to write? I could give a Top Ten list from books that haven't been published...primarily because I haven't nailed certain pivotal characters, but that might be cheating.
So, I'll give you the character whose reactions to situations I most often have to rewrite.
Feng, the Phoenix from my Immortal Spy series. Feng is introduced as the antagonist to the protag. Feng is painfully righteous, morally pure, and utterly inflexible. As events unfold, things don't go well for Feng. Things go so poorly that even the most gruesome death would definitely be preferable. Alas, to his great misfortune, he survives thanks to the protag. While his body eventually recovers and he finds common ground with the protag, he suffers from horrific PTSD.
What makes Feng difficult to write is that his PTSD doesn't (and shouldn't) define who he is, but it is a factor in his reactions to situations. It is a question of his reliability in a crisis. What will trigger an episode? How will he respond? How do I responsibly write his management of his mental health without pandering or being dismissive? Yes, I know, I'm writing fantasy...about a dude who is an all-powerful firebird. Yes, the story is told from the POV of protag with her own mental disadvantages. Still. It is some bad shit when your mind is simultaneously your greatest asset and your worst enemy. Now, I'm not saying I get it right. No, no. I am saying what makes Feng so difficult for me to write is my desire to at least be in the neighborhood of respectful.