Who is Kokushibo In Demon Slayer?

Kokushibo is a well-known demon, regarded as one of the fiercest to appear in Demon Slayer. Despite all that is known about him, he is nonetheless enigmatic. He is regarded as one of the most experienced demons in existence and has engaged in several combats. He prevailed most of the time, but even after hundreds of years, he was unable to let go of the issues from his human past. Ultimately, these issues turned him into a devil and brought about his downfall.
Tanjiro, the protagonist of Demon Slayer, is a young man who trains assiduously to become a demon slayer in order to discover a cure for her loving sister Nezuko, who became a demon herself. In order for her sister to lead a normal life, Tanjiro desires to transform her back into a human.

Kokushibo As a Human:

It is said that Kokushibo hunted demons hundreds of years ago. Although he was strong, his twin brother developed a unique combat technique and was much stronger. Envious, Kokushibo created his own style, yet it was insufficient to ease his feelings. Even after he transformed into a demon, this jealousy stayed.
Kokushibo had unique training from his younger, incredibly powerful sibling and even received a mark that improved his abilities. There was a catch, though: he would pass away young because of the mark. Kokushibo, fearing death, took a startling step to extend his life: he transformed into a demon. He became one of the very few demon hunters to change allegiance as a result! He didn't harm his wife and kids even though he turned into a monster.  

Kokushibo As a Demon: 

After turning into a demon, Kokushibo had a spooky transformation. He developed additional eyeballs and was much more frightening. He also developed into one of the greatest swordsmen. He claims that his methods were so well-honed and renowned that there is little chance they will be preserved for use by subsequent generations. Living as a demon, he escaped the curse that would have killed him if he had been a human, and survived for over 500 years. He overcame several opponents as he rose through the demon ranks to become the strongest one. He could have been eating these losers since he even has a unique capacity to absorb some of them. Akaza was the only opponent he spared due of his fierce rivalry.

Kokushibo eventually defeated his sibling despite being an extremely powerful demon. But he had not expected to win. His sibling, it turned out, had escaped the death curse and lived a long time. This caused Kokushibo to doubt his decision to turn into a demon to live forever. It isn't clear to the audience how much this doubt troubled him until his passing.