It kinda irks me when people call Black Mask "Truekechi", because that's not really the case...? Not entirely, at least.
I mean yes, of course, Black Mask IS a part of Akechi's personality, I'm not denying that. The thing is, that it isn't Akechi's WHOLE personality. This kid just went through the biggest crashout of his life, his whole plan he's been meticulously working on for years failed, and the worst part is that it didn't even lead to what he was after, so he didn't even get the revenge he wanted. Of course he's unstable and edgy, I mean, who wouldn't be?
That is not to say that his anger and edginess aren't a part of his "true self"; I'm sure that his glee at taking out shadows is absolutely real, for example, but I believe that it comes from him just being angry in general- at Shido, at Maruki, and at himself. Akechi is rough around the edges, has a short temper and has a tendency to look at others from above, but the situation he's in boosts these parts of his personality as a sort of defense mechanism.
I think that his mementos conversations give a really good insight into this too; Perhaps it's due to my older age compared to the characters in-game, but a lot of Akechi's lines feel so... Forced on his side? He's the one to bring up that other Thieves probably don't want to have him on the team, or that they're unsettled by how he fights, but nobody even said anything about it in the first place...? Even other characters seem kinda done with his act, either that or they accept him being on the team due to their shared goals; Some are even quite nice and friendly, all things considered. This just feels like Akechi's self-loathing twisting itself into this kind of quips because he himself doesn't want to get too comfortable on the team; He's scared of growing closer to the other Thieves, so he tries to keep them at arm's length. He can't just tell them "I think I'm a horrible person" straight up (his pride wouldn't let him), so he has to make himself "unappealing" in other ways.
There's also the other side of these conversations, where you can see him just... Being a kid. He whines about being hungry, or thirsty, or his legs getting stiff in the car, he talks about mundane things with other Thieves like graduation or sports... He's just a teenager as much as his teammates are- it's just that the others weren't hired as assassins at 15 and didn't grow up obsessed with the plan to kill their father. His emotional development and maturation process have clearly been impacted, on top of everything else he had to deal with. So he may act "adult" and more mature than others at first glance (at least before he reveals Loki), but inside he is just as much (if not more, in some aspects) of a kid as the other Thieves are. This means that he is still at the age and mental development level where he is still figuring out himself, who he is, what path to follow etc. It also means that he is prone to being overly dramatic, emotionally extreme, and have poor emotional control- you know, teen stuff.
The key to it all becomes his ultimate persona, Hereward. We all know that Akechi has two personas, but he doesn't get two ultimate personas, he gets only one. I think that it's symbolic of his psyche slowly melding itself into one again. It's not that either one of his "sides" was truly fake or real; Both the Detective Prince and Black Mask ARE parts of the "real" Akechi- it's just that he has been systematically compartmentalizing his whole self into two for YEARS. The awakening of his ultimate persona is him starting the process of healing and allowing these two sides of him to mix and reconnect back into one.
I honestly think that, with enough time and the right environment, Akechi's most extreme traits would even themselves out, and he would stop being AS short-tempered, snarky, and blood-thirsty as he is in the game. He will never be the perfectly plesant Detective Prince, but he also won't be the angry, self-loathing and vengeful Black Mask. He will be something new, something different, a fusion of the two opposite ends of his personality spectrum. Only then he will live as his fully true self.
