I think that a lot of people see Hereward's design as kind of boring and uninspired, but tbh I think that's kind of the point.
Akechi's whole character conflict is about wanting to be the kind of hero of justice he dreamed of being as a child (symbolized by Robin Hood) and his disgust towards society and what he learned about it thanks to his rough childhood (symbolized by Loki). He can't fully commit to being a hero of justice while knowing so much about the dark underbelly of society (and after everything he's done that contrasts with that squeaky-clean, one-dimensional image of justice he had as a child, oblivious to the evils of the world), but he also can't fully let go of these naive, childhood ideas and just fully plunge into that darkness he has in his heart.
Hereward is explicitly the result of Robin and Loki merging together, which symbolizes to me that Akechi has managed to find a way to somehow let these two desires, those two sides of himself, finally come together into one coherent sense of self, all thanks to his relationship with Joker.
I wanted to highlight just how "practical" Hereward's design is. His armor looks like irl tactical wear, the extra pockets by his belt are good for storing extra things, his boots are high and armored... Even his bow just looks like a regular modern compound bow, which gives it a much different, more realistic feel than the one Robin's fanciful bow has. It's a very simplified design compared to both Robin and Loki- Hereward is almost entirely black, with no extra colors or patterns outside of the RH on his chest, his (relatively small) cape and the extra wings on his helmet. It's a very no-frills character.
I think that Hereward's design can give a very interesting glimpse into Akechi's mind after the 2/2 conversation with Maruki and Joker. Hereward is heavily reminiscent of Robin when it comes to build, but his dark palette seems to be an influence from Loki- Hereward has obvious visual influences from both very different personas.
Akechi decided that he wants to be a hero of justice like he wanted to be as a child (which is why Hereward looks so much like Robin). However, now that he grew up, learned about and experienced the darkness of this world, he can no longer follow the squeaky clean version of justice he believed in as a child- that vision had to adapt and incorporate both what Akechi saw and experienced with others, and also what he himself did (which is why Hereward seems "darker" and more inspired visually by Loki).
Akechi's knowledge of the darkness in society and of the one dwelling in the human heart has allowed him to take that fanciful idea of what being "a hero of justice" means to him and to make it into something more... Pragmatic. Hence why Hereward wears a realistic armor and uses a realistic bow, compared to Robin's bright tunic and his golden, impractical bow, but also compared to Loki's patterns and huge sword, which are, imo, just as very theatrical and "extra".
Speaking of Loki's patterns, Hereward is almost completely black. Loki's patterns are based on dazzle camouflage meant to make battleship's actual size and dimensions less clear from a distance. So it is a form of camouflage, but not the kind that is meant to mask whatever's covered in it; Quite the opposite, it's in the name- it makes the wearer look extra, flashy, it draws attention. Hereward being just one tone, and that tone being black, seems like a kind of admission that Akechi doesn't revel in his cruelty or "evilness" anymore. Black is a color used to blend in with the darkness, it signals the opposite of wanting to be seen, to be witnessed- Akechi isn't interested in drawing attention to himself anymore, not for being a hero of justice or by being the irredeemable villain.
Given that Hereward is created once Joker promises Akechi that he will tell Maruki to destroy the false reality, I suppose that the conclusion is that, by respecting Akechi's wish, Akechi finally feels seen and "loved" (in whatever way you want to interpret that). He doesn't need to fight for anyone's attention anymore, not by playing a hero or the villain, because he has finally been seen and accepted by someone who knows about both sides of him. Joker doesn't idealize him nor does he see him as someone entirely, purely evil, which means that he accepts and "loves" Akechi as a whole, and as an equal, nuanced human being. Akechi can finally stop desperately seeking attention and love from the whole world, because he finally is "loved" by someone he respects and cares about, and in the way he always desired.
Anyway, circling back to Hereward's practical, pragmatic design. I think it also symbolizes Akechi applying his experiences, both good and bad, into his fight for justice. He still has the same goal as he had as a child (I think it's even a source of some pride for him, given the prominent, vibrant RH on Hereward's chest), but his experience allows him to recognize that justice isn't always black and white- now he can recognize nuance and knows more practical ways he can bring that justice to fruition, since as a child he lacked the knowledge and experience to do anything but to dream about it. Persona's whole main theme across the whole franchise is growing up and maturing, and I think that Akechi illustrates it very well, since accepting nuance and grey areas in life is something that's associated with teenhood and maturing.
I also think that it represents that Akechi accepted both sides of himself, both his childish side that desires justice, even when the real world is often unjust and complicated, and also his angry, "ugly" side that he developed when he was trying to navigate his life alone and without guidance, which pushed him into doing immoral things. It's like he's saying "Yes, I can be childish. Yes, I've done horrible things. But I recignize and I accept both of these sides of me, and I will use them to my advantage to make the world a better place in a way I see fit, because I'm tired of trying to appeal to what others see me as and want me to be. I don't have to do it anymore, because I've finally been seen and recognized as I am and accepted for all I am and can be one day".
(It's interesting that Raoul also somewhat follows the same design cues. Arsene's feathery wings become mechanical, and he ditches his historical clothing for a more sleek, modern outfit. I think that it reflects the experience Joker gained during his time as a Phantom Thief, which allowed him to take his vague, "ingrained" (symbolized by the biological birdlike wings of Arsene) desire for rebellion to and gain knowledge and practice through his experiences with the Metaverse. Raoul's wings seem more like gear he had made because he KNEW he'd need them for infiltration, and not like an inherent trait that his persona just has naturally, which to me implies gaining knowledge and problem solving. I like that he keeps his stilettos tho- because he's just as much of a show off with a penchant for theatrics as he was at the start.)
