
Akaza is the best demon in Demon Slayer
He is so well-written; at the very least, he is one of the best-written in the whole manga.
Hakuji was a kind and caring human who was only trying to do the right thing. He took care of his dying father, and though he stole and fought to acquire the medicine his dad desperately needed, it was out of selflessness. Even after his father’s eventual suicide left Hakuji with no one and filled him with resentment towards the world, Hakuji was still able to start over again.
Hakuji was taken in by Keizo, the man who would become his master. Hakuji would also go on to care for his master’s sick daughter, Koyuki, whom he fell in love with and was to marry. Through his training with Keizo and caring for Koyuki 24/7, Hakuji’s heart found salvation, and it seemed as if he would finally be able to live a normal life, as his father wanted.
However it all came crashing down years later when Keizo and Koyuki were murdered by students of a rival dojo. The reasoning for the rival dojo’s horrendous act of poisoning Keizo’s well was beyond petty: they could not beat him or Hakuji in a fair fight. As was to be expected, Hakuji lost it and went on a rampage, wiping out all but one of the rival dojo’s people. It was right after the massacre that Hakuji was found by Muzan and became a demon.
Once becoming a demon, Hakuji, now called Akaza, subconsciously built his entire identity and beliefs off his past human life as Hakuji. Akaza never ate or killed women, a trait created from his subconscious memories of Koyuki. The color of his hair, eyebrows, and vest became part of the same color palette as the kimono Koyuki once wore. His compass needle technique became a snowflake, and Koyuki used to wear a three-snowflake hairpiece, with the latter part of her name containing the kanji for “snow” (yuki). Akaza’s obsession with strength came from his promise to Koyuki to get stronger and protect her for the rest of his life. Even his attacks and techniques were named after fireworks, again, his subconscious memories of Koyuki.
Despite being corrupted by Muzan and losing his memories, Akaza still remained deeply intertwined with his past life. That is how strong Hakuji’s love for Koyuki was. It was also that strong love that saved Akaza in the end, allowing him to finally be at peace with Koyuki, albeit in hell.