Akaza

Akaza is by far the best-developed character in the series. All the Oni had traumatic pasts, as is clearly shown throughout the series; before becoming demons, they were once human. Akaza was no different. The brutal murder of people who were important to him left him confused, aimless, and hopeless. Unfortunately, what welcomed him in that moment of weakness was the evil power of Muzan, corrupting him and transforming him into one of the strongest Onis in history.

As a human, Akaza saw those he loved taken from him not by stronger enemies or his own weakness, but by weak, cowardly men who did not have the courage to face him with their own fists and resorted to a cowardly measure: poison. For him, this was the height of injustice. It wasn't enough to lose his fiancée and father-in-law; he lost them to what he despised most, to cowardice hidden behind weakness. Since then, this act has become a symbol of everything that is base, of everything that shies away from direct confrontation. And that is why, as a demon, even after forgetting his past, everything that happened in his human life, this was something that marked him so deeply that he still clings to hand-to-hand combat and also to a certain virtue of dueling, almost as a way of exorcising that invisible grief. As if each fight were an affirmation that his true strength only exists when it is put to the test, without artifice or tricks.

There is indeed a little virtue in Akaza compared to the other Oni. Now, what he didn't realize or forgot, and what made the demon even more twisted, was that all this hatred was not against the weak, it was against himself, his human past. He, again, was not there to protect, and even if he had been, there was nothing he could do. All the cards of Akaza's destiny were dealt as if he had no free will, there was nothing he could do, he had no way of knowing that the water was poisoned, nor could he cure his own father. It seems that all of this was premeditated and he had to accept the events and not fight against them. He could even fight against the events, but that would frustrate him anyway, because he would not be able to solve them. These are things that do not depend solely and exclusively on his strength, but involve a number of other factors.

Akaza's end is sad because, after fighting so hard against those he thought were weak (humans), he realizes that he was weak because of the paths he took. In a moment of epiphany, remembers what he was and what he has become, punching his own body, punishing himself for the path he has taken.

Honestly, I wish this character could have been explored even before the events of Infinite Castle. Before the movie, his appearances are brief and short. Still, a very well-written character, by the way.