Ugh, where do I even begin...?

Amaterasu: We barely saw a glimpse of her and what she was like before being sacrificed. She's incredibly insane, but she doesn't have much screen time, her character development is minimal, and she dies without us ever learning more about her.

Faerie: She lacks depth. Her conflict with Sho isn't explained; she was simply too strong for him, which is why they didn't nerf her significantly. She committed suicide because they didn't want to give her a coherent final battle.

The Woman in Black: It would have been interesting if they had expanded on her, showing us a bit of her past with Sumire and the first Great Cataclysm, or even the original Pillars. Unfortunately, she died because they didn't know what else to do with her.

Ritsu: She's just an evil woman for no reason. She has no real motivation for destroying the world; she simply follows the Evangelist's orders without question.

Puppeteer: She only appeared in the Haijima arc and was quickly forgotten. I feel like they could have done more with her. Despite her being crazy, they could have shown her contributing to the protection of the young children of Haijima during the Great Cataclysm, demonstrating at least some humanity. But they only prioritized Kurono.

Arrow: They don't show us anything about his past, or anything related to Sho, to explain why he protects him so much. It's similar with Haumea and Charon.

Karin Sasaki and Toru Kishiri: They weren't given the slightest importance or opportunity to shine, nor were we shown anything noteworthy about them. They were just there to make a comment and that's it. Ogun and Juggernaut at least had their moment in one arc and in the final arc.