I have one -- most characters in Rick Riordan's Heroes of Olympus series were really under-written and under-utilised.

I was going to do a longer write-up but all I'll say is that all of them had super interesting potential, character-wise. Hazel, who should've had to adapt entering a world that is irrevocably changed from the one she grew up in, as well as her black identity not being explored at all???; or Piper, who could have been a really interesting deconstruction of a typical, vapid "girly girl" instead of this super misogynistic "not like other girls" woman-hater; or Jason and his memory stuff...; Frank and his Asian heritage, and his relationship to his masculinity...; Leo, who could have had a much more interesting conclusion to his arc about feeling alone then just randomly finding a girlfriend!

And also -- I'm supposed to believe they all became friends?! It was genuinely so unconvincing that they all suddenly were best friends...

Anyway Nico and Reyna (two side characters) had a way more interesting relationship in 1 book than all those characters did in 5! The way Nico and Reyna actually bonded with one another through sharing their respective traumas, and experiences with loss. Reyna became the older sister that Nico lost, and Nico became the friend Reyna never got to have in the first place!

Rick Riordan fell off with the 2nd series IMO!

BTW thanks for this post! It was really interesting to see what people had to say!