Ao no Exorcist has this really dense internal conflict baked into its premise that a lot of fics just skate over for fluff or action, which is a shame. The best ones for character development, for me, are the ones that let Yukio simmer. Everyone focuses on Rin's explosive emotions, but Yukio's the pressure cooker. Fics that unpack his dual life—the perfect student versus the exorcist who was basically raised as a child soldier—hit hardest. There's an older one I can't recall the name of, where after the Impure King arc, he has a complete, quiet breakdown alone in his dorm room, and it's not about someone finding him, but about him slowly putting himself back together with clinical precision. That stuff sticks with you.
Crossover fics can be surprisingly good for this too, if they're the right fit. A 'Fullmetal Alchemist' crossover where the Elric brothers meet the Okumuras and recognize that same flavor of childhood trauma traded for power works because the themes align. It forces both sets of characters to articulate things they'd normally swallow. Those fics are less about the fight scenes and more about the conversations afterward, which is where real development lives.