I recently just finished the series, and I feel like Kayano's story was done so half-assed. Personally, I think even if the twist came out of nowhere (which by itself is a huge disservice to the character), it still wasn't fleshed out or explored enough after the reveal. I wish we could've seen her slowly shift her opinion. In fact, I'd listen to Takaoka whisper in my ear with his weird dad/family shtick if I could see the series from her perspective (i.e knowing she's in class 3-E for a different reason than the rest of her classmates, continuing the series lighthearted format with slow reveals about her true motives (flasbacks, etc) and feelings shifting as she sees the growth of people (she originally thought would be like props to her) around her because of koro-sensei and care for them as well (making it an emotional back and forth (maybe with a bit of sabotage here or there)), which I think would've made her confrontation with korosensei feel like a huge pay off to her charecter development + the revealtion of aguri's death + her acceptance back into the class being a lot more impactful.)
ALSO, I hate, HATE, HATE how the battle between her Korosensei ended with Nagisa's kiss. It was such an anti-climactic end to a scene with so much potential, which also further contributed to Kayano's character being focused on having a one-sided crush on Nagisa for the second half of the season. Plus, I feel like her having a mentor-like relationship with Korosensei (like him with Nagisa, even if it's slightly different because she doesn't become a teacher), could create that early tension and have her fleshed out after the reveal (like she barely discusses her sister with him afterwards, imo (would've listened to Takaoka AGAIN for a one-on-one scene about this),
ALSO (again), (not talking about Kayano anymore), Itona, I personally feel like, was also so underdeveloped. His storyline was so interesting, but after he joined E-class and Terasaka's gang, he had barely any screen time. He never expresses any grievance or feelings about being manipulated by Shiro, which I think could have had a payoff in the final fight.
Obviously, I understand that not every character can be explored equally, but I would rather have had Kayano or Itona developed more instead of wasting screentime on yet another porn joke/scene.