Akaza

Akaza, born Hakuji, is a major antagonist in the 2016 dark fantasy manga/anime series Demon Slayer: Kimetsu no Yaiba. He was the Upper Moon Three and the first demon to be promoted into Twelve Demon Moons by Muzan Kibutsuji.

He first appeared as the final antagonist of Demon Train Arc and considered Tanjiro as his personal rival and grew resentful towards him. Later, he fought Giyu and Tanjiro in the Dimensional Infinite Fortress to his death, but not before he managed to recall his past and took his own life as an act of remorse.

He is voiced by Akira Ishida in the Japanese version of the anime. Ishida's other villainous roles include Zeref Dragneel in Fairy Tail, Gaara in the Naruto franchise, Gabriel Miller in Sword Art Online, Regulus Corneas in Re:Zero − Starting Life in Another World, Hyoga in Dr. Stone, Ryuunosuke Uryuu in Fate/Zero, and Hideo Shimada in Parasyte.

Biography
Go see on the villains wiki.

Personality
Akaza is a stubborn, harsh, short-fused and blood-thirsty fighter who desires to fight the strong, respects the strong and loathes the weak. He also shares ill feeling to his colleagues and sees them as his own rivals, being especially hostile towards Kokushibou, the Upper Moon One, and Douma, the Upper Moon Two, who both held higher ranks than him. He shows a particularly strong interest to powerful opponents and will try to figure out their name. As a result, he remembered the name of every opponent he fought as respect towards them.

In spite of his cruelty and wrath, Akaza had one special code; he never devoured any women, in spite of the fact that eating females would have actually made him even stronger, and Muzan gave him special permission to do so. He also showed a serious self-loathing tendency, valuing others above the good of himself.

All of his personality was influenced by his past life and the remnants of his own humanity, showing how dark and miserable his traumatic experiences were, to the point of being forced to become demonic and joined Muzan.

What Made Him Redeem Himself

 * Akaza was originally a good man, but he lost all his loved ones for ridiculous reasons and was forced to become a Demon by Muzan.
 * When he returned to hakuji with all his memories as human beings, he apologized for failing to protect him with koyuki and the master, and fell into hell because of his sin as akaza, but koyuki went to hell with him.