I disagree. He was my favorite character (Futaba was my other one so I understand where you're coming from) as had the most depth, and also since they insist on making the protags silent-ish most of Joker's character development was also done through comparisons between him and Akechi. By most depth I mean that he develops the most as the story goes on- other characters get kinda frozen after their arcs because for a story oriented game persona has a fatal flaw in that hanging out with the main cast and seeing their individual stories is optional and so can't be referenced later. So right off the bat he just makes the story more interesting with those two things.

One thing I really like about his plot relevance is that what happens on Shido's ship is the first big sign to the thieves of what they're about to see in the depths of mementos. Targeting individual corrupt people is not going to change society like they want. People were more interested in the spectacle than anything and are willing to just ignore whatever isn't interesting anymore. I know it's explained as a god making everyone not care about what Shido did but all these gods come from the human psyche in these games anyway so. After Akechi sacrifices himself for them the thieves go on about how he was "a victim they couldn't save" which is pretty much it- their naive lib child way of going about things will never address the system that failed Akechi and tons of other kids like him. He would've been around 15 when he started working for Shido and the guy figured out that was his son and used that to groom him as a hit man, it's pretty messed up. Doesn't take all the blame off of him but if there was any justice I'd want a kid like that to be rehabilitated. As well as his baby's first (horribly thought out) revenge plan it was also his ticket out of foster care to be in that situation. In addition to all that it's a reflection on how being so isolated from community and being sold the pull yourself up by your bootstraps little orphan mentality really hurts people and is ridiculous to expect.

You're also slightly off about the circumstances of Wakaba's death. Akechi didn't control the where but that's still his fault of course. But the suicide note was done by shido's men not him. It was absolutely to make sure she was never a threat because she was so traumatized yeah. I actually like the theory that Shido is also futaba's dad bc considering the circumstances it's possible unfortunately.... He's also probably kinda guilty about what happened to Futaba because of how he acts around her when he's in the cafe, how he talks to sae about her unethical threatening sojiro, as well as in the anime they have him step in to her aid at one point. Again doesn't change what he did but he's not hopeless as a future adult who has learned better.

Haru's dad idc he wanted to sell his daughter off as a sex slave and also he was anime jeff bezos have fun in hell lmao I think Haru should've shown growth as a character and had issues with Akechi because he stole her choice away in the matter of her father, instead of being like oh no im sad about daddy. She chose to change his heart, which is also a cruel fate so it must've been hard for her, and that was just taken. Kinda sucks.

I also really liked Royal's story and his role there but I don't feel like writing any more than this right now lmao