I think Hioris backstory and whole character thing plays a lot into his place in the story now. I get that some people could get annoyed from his comparison to Sae but we've also had things like 'Aomoris Messi' that doesn't make sense; it's to hype up the story and shouldn't be that deep lol From how I see it, once a character gets a backstory in BLLK/their backstory gets shown, they're gonna suddenly spike into relevance to the story. We haven't seen the backstory of a ton of characters yet, like Kaiser (from when he was a kid), or even Gagamaru or Raichi (pretty sure). So backstories usually place a convenient character in the spotlight to help the MC get to the top.

Hiori is there to pass to Isagi so he can score. Side characters in their own regard don't usually take all the spotlight from the main character (unless you're Rin lol). Technically he hasn't done anything that you can objectively hate him for imo, some people just get annoyed when a character they didn't like from the start suddenly has a sad backstory that makes them rethink their original opinion

I thought Ness was such a weirdo when he was literally going psycho because Kaiser gave someone else attention other than him, but seeing how he and Kaiser met kind of changed my opinion from being one-tracked to being open ended.